What is the current estimate for the total number of species on earth, and why is it so hard to get an accurate count?

The current estimated total number of species on earth is between five million and hundred million.

The reason why it is hard to get an accurate count is because some species are very hard to find reason being that of nocturnal habits, location, and the size.

All the species which have ever lived on earth are being estimated to be extinct. It is approximately one extinction for every million species for years. Mammals species like typically persist for one million years.