It's made up, but definitely a better estimate than both 2^100 and the current number of known species. I imagine one way to obtain a coarse estimate would be fitting the number of known species over time to a curve like N-e^-kt curve and then using N as the estimate. Coarse, but not terrible if you can quantify the expected error.
People have come up with vastly different answers to this question. That to me heavily implies "i don't know" is the only right answer at this time.
"Researchers have come up with wide-ranging estimates for how many species there are. As May points out, this ranges anywhere from 3 to well over 100 million – many orders of magnitude of difference. Some more recent studies estimate that this figure is as much as one trillion."
