I'm unaware of even an unofficial estimate. They are widespread and certainly not an endangered species.
there is only one in the world
The praying mantis (Mantis religiosa) is not endangered anywhere. There are many other types of mantises, sometimes called "praying mantises" colloquially, around the world though, and some of them are endangered.
Praying Mantises eat many small insects, including grubs.
yes, and no at the same time. There are some extremely endangered species of praying mantises as of 2011. Yet there are still many species that are thriving in the eastern united states and in Australia.
A praying mantis egg sac typically contains around 100 to 200 eggs.
the praying mantis eats all of the slugs and other bugs such as ants and pests that eat your flowers and plants.
They lay about 3-4 eggs in a month after maried.
Many insects breathe through spiracles, holes in their exoskeletons. See the related link for more information. Red blood cells collect oxygen that happens to enter the tubes. This is a passive form of respiration, so oxygen is not actually pulled in, it just happens to be there. Most insects breath this way, and that is why they don't get very big.
last year the mantises must have laid more eggs and over 200 mantis come out of one egg!
Estimating the exact number of praying mantises that die in a year is challenging due to their widespread distribution and varying life cycles. In the wild, many factors such as predation, environmental conditions, and disease contribute to their mortality. Additionally, the lifespan of a praying mantis can range from a few months to a year, depending on species and environmental conditions. Therefore, while it's difficult to provide a specific number, it's clear that millions of mantises die annually in natural ecosystems.
female mantids lay up to 5 oothecas each ooth containing 200 to 400 eggs but only a quarter will survive till adulthood
Dozens at least, maybe more.