There is no set number for all species. This is because each species has its only population dynamics. Different things factor into estimation of the status of a species including how many are capable of actively reproducing, birth rate, death rate, available habitat, and possibility of external threats like predators or natural disasters.
For example, 9 of 31 Przewalski horses that were captive in 1945 are the ancestors of all of the 248 living individuals today. http://en.wikipedia.org/wiki/Przewalski%27s_Horse
But if you were talking about an insect, say a butterfly, 10 individuals or so may not be enough to recover to a “normal” size population. In these two cases, the butterfly might have to be considered endangered with several hundreds or thousands individuals left whereas for the horse you may be able to wait until there are a few hundred left.
Then you have to go into things like minimum viable population and effective population size. If you are interested in learning about these concepts, check out the links below.
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