No, there is no documented "unhealthy" limit of seamen someone can ingest orally, vaginally, or anally.
for the taste of it
Some people ingest semen during sexual intercourse.
well no not really it is protein lol Answer: you can and it wont hurt you.
The rifampin in the semen will not cause any problems, but their semen may not be safe to ingest. Rifampin is an antibiotic given for many reasons, including STD's. If the rifampin is being given for an STD then it would not be safe to ingest the semen, not because it has the rifampin in it, but because it may still have infectious particles in it.
Girls do not produce semen, but yes, if you are free of diseases, another girl can safely ingest your secretions.
No
Full of vitamins ingest away, assuming no std's aids etc.
In average 3ml / ejaculation.
Semen is the total ejaculate. The sperm content in the ejaculate is a tiny part (about 5% of the total volume, the rest is lubricating fluids and nourishment for the sperm while it is on its way. Semen content and semen volume would be two different things & your question is ambiguous. If the sperm content of the semen is low you wouldn't be able to conceive children. If the volume is low, you just wouldn't have much ejaculation. you could still have an orgasm, just wouldn't be much semen there.
17 gallons
Seninal Vesicles
Most Islamic scholars frown on oral sex as a general matter, so there is rather firm opposition to swallowing semen. Of course, what Muslims do in the privacy of their own homes is a different matter entirely.