The number of known living species on earth sets somewhere below 2,000,000. A review of the literature shows that in the 1970's, estimates of the actual number of extant species ranged from 10,000,000 to 100,000,000, with 30,000,000 being a reasonable average. In this post-genomic era, with bacterium included, and extrapolating back 3.5 billion years knowing what we now know about the ratio of extinct to extant species in some families, an astonishing 4 trillion species of life can be conjectured to have existed over geologic time.

unrelated, there's an estimated 5-8 million species of beetle alone.