At best, a mare can have a foal every from age three to age 25, so about 22. Only the very best producers will have more than about ten.
Male horses become sexually mature at around two years of age, and are able to produce a virtually unlimited number of foals. In a wild herd situation, a young stallion usually establishes a harem of mares for the first time when he is 4-5 years old, and is able to maintain a harem until he is about 12-15. Depending on how strong he is, and how well he competes with other stallions, the harem may number between 3 and 30 mares. He will father a foal with most of these mares yearly.

In a domestic situation, where humans control which mares a stallion mates with, the answer varies hugely. He may have no foals at all (if he is not wanted for breeding he will usually be gelded) or he may have hundreds or even thousands - a stallion can mate up to three times a day without losing condition, and modern reproductive technology is able to extend semen so that up to 6 mares can be bred from one ejaculation.

A mare becomes sexually mature at around 3 years of age, but a responsible breeder will not put her in foal until her bones have stopped growing at 5-7 years depending on her breed. Her pregnancies each last a little more than 11 months, and she can be bred again as little as two weeks after foaling. Therefore, she can produce up to one foal a year, and depending on her health, may continue to have foals until she is around 25. Theoretically, therefore, a mare bred every year from the time she is mature until she is too old to breed could have almost 20 foals.
Normally only one a year... though twins do happen occasionally.

A horse can have up to 2 foals, however, twins are very rare-1 in 10,000 and 80% of those die. So live twins are very, very rare.
A healthy, well-managed mare can produce a foal every year into her twenties.

You'll know that a mare is having trouble foaling if she lies down and strains hard for more than half an hour with no sign of her labour progressing. If your mare is having difficulty the most likely reason is that the foal is not presenting the right way. A vet is trained to deal with malpresentations, and it is very difficult for a layperson to fix them.

If you can't afford to call a vet, I hate to be blunt but you shouldn't be keeping horses, let alone breeding them. If the vet can't come or is a long way away, you need to try to help the mare because you may lose mare and foal if you don't intervene. Tip some disinfectant in a bucket and disinfect your arm. Tie the mare's hind feet together so that she can't kick you and feel inside her vagina.

If the cervix is open, try to find the foal. If you can feel a head, it's facing the wrong way. The head should be lying on the front legs, which normally stretch out along the vagina. If the front legs are folded back, the chances of the mare being able to foal are very limited. You will need to disinfect some string. Slide your hand back along the foal's leg and tie the cloth around the pastern. Leave the end of the string hanging out of the vagina, and ask somebody to pull firmly on it while you push the head back into the uterus. This pulls the legs around and after that the mare may be able to foal normally.

If you can feel a tail, the foal is presenting breech. Most mares can deliver a breech foal unless it has its legs back - use the string around the pastern again, and this time push back on the hock to pull the leg around. If the foal's head is back, this is a virtually impossible presentation. You can try to pull it around, but often this won't work.

The only thing you could try would be to shove the foal as far back as you can because there is more room to try to move the head in the uterus than past the cervix. If the foal is jammed or presenting sideways, shove it back as far as you can and then try to realign it so that it can come out normally. In a small mare where the foal is stuck or too large to fit through her pelvis, there is nothing you can do except a caesarean operation, which is obviously a job for the vet.

If nothing has any effect and the vet is several hours away and the mare is obviously suffering, it might be a case to make the very difficult decision to put the mare down. Equine obstetrics involving head-back, stuck foal, sideways presentation or coiled breech usually require either expert handling or a caesarean operation. If you have to shoot the mare, there is a chance that you may be able to save the foal. Cut through the skin of the flank. The uterus should be the largest organ and will bulge into the cut. It would probably be possible to remove the foal.

When a mare has trouble foaling, it means that something is wrong. It's a job for the vet. All of these above methods are LAST RESORT ONLY. If there is a choice between shooting the mare and trying to foal her, that's when you try something like this without expert assistance.

A bull is a bovine and a mare is a horse. Your question makes no sense.

It depends on what part of the word "dangerous" you are referring to. Are you asking is it dangerous for the mare or dangerous for the bull?

From the gist that I'm getting from the question asked, I have to say yes, especially for the bull, as he could get injured if he keeps trying to breed her. Please do these two animals a favour by completely separating them from each other.

Additionally, though I have no idea what kind of other animals you have on your farm, if you have the bull because you only have one or two cows, do yourself (and even the mare) a favour by considering selling him. That bull is most likely attempting to mount the mare because he is sexually frustrated that he doesn't have a large number of cows to breed with. One bull can be used to cover from 10 to 50 cows or heifers in one breeding season: this means that cattle are polygamous, not mono- nor bigamous. If he isn't given the opportunity to release his pent-up sexual energy by being with his girls for a few months to breed as many of them as he can in that period, then he will find other ways to release his frustration and attempt to breed other animals like your mare, or break out of his enclosure and go in search of other cows that are requesting his services.

Bulls can also be very dangerous, and cost more (especially in feed and mineral) to keep than it costs to keep a couple of cows around and breed them via AI. (AI is an alternative and completely harmless to having a bull around, especially if you only have one or two cows around.) It's also not worth anything by keeping a bull around as a "pet" (bulls are actually the worst kind of pet you can have) if you have no cows to speak of and if he's causing more trouble than he's worth. If that is the case, then consider selling him to someone who can make better use of him.

If you can't or don't want to get rid of the bull for whatever reason you have, be it that you're keeping him because you are some sort of farm sanctuary, or you cannot get any local artificial insemination technicians out to provide regular breeding services to your cow[s] (or don't wish to have your cows serviced via AI because you believe it "cruel" and "unnatural," the former which is not and the latter is), then keep the bull and your horse separate from each other permanently. I would also strongly recommend you consider the future of this bull from what I mentioned to you above if he is causing more trouble than just trying to perform interspecies coitus.