Answer:
The number of species of lemur recognized varies depending upon which expert you talk to. The number ranges between 50 and 101 (as of 2010). Some experts suspect that more species are yet to be discovered, possibly bringing the total to 125. The new species that have been described are actually sub-populations of existing species, but various tests have demonstrated that they are distinct from other populations. But again, this is debated.
As for the actual number of lemurs (as individuals), no one knows. Some species have as few as 100 individuals remaining, while others are fairly plentiful, yet still in decline due to habitat loss.