According to Robert May, this would be the first question an alien visiting earth would have. Besides the pure curiosity, that such aliens might have, this is quite a relevant question for us humans already here on earth due to the ongoing biodiversity crisis. That is the extinction of more and more species in the course of time. So how many species are there now on earth? What would you guess?
A recent study predicts a diversity of around 8.7 million species globally with an uncertainty of ~1.3 million. Roughly 2.2 million of these estimated distinct life forms are marine. This study also claims, that 86% of the existing species on earth and 91% of the oceanic species have not yet even been described. You will easily realize that the amount of different species on earth is even bigger than this estimate. With the ongoing extinction on the other hand, 18.788 species out of 52.017 so far assessed are threatened, an accurate number is, thus, impossible to give.
These results raise even more questions: What do we actually know about life on earth and how is a species defined in the first place?
 May, R.; Science, 329, 2 (2010) p. 41-42
 Mora, C.; PLoS Biol., 9, 8 (2011) e1001127