Any number is a bad guess because we don’t have any sense of the likelihood of life emerging, even in the “optimal conditions”. It could be so unlikely that it’s probable there is no other life, it could be so likely that almost every “viable” planet has life, anywhere in between, or it could be that God created exactly as much life as he wanted, which could be, again, any number.
We have a sample size of one and what must be, in the grand scheme, a primitive understanding of physics and chemistry and biology. Throwing around numbers and probabilities for the existence of other life is frankly ridiculous. The best we can intelligently talk about is what the number is given some guess about other numbers. But remember, we have no good numbers for the emergence of life, the survival of life, the likelihood of evolution into intelligent life, etc. Any number multiplied by a totally unknown factor can be literally anything.
And before someone throws the infinite universe argument at me, of course. Any probability above zero times infinity is infinity. (Though it’s also possible that there is still no extraterrestrial life even in that case, that outcome always has a non-zero probability!) If we accept a probabilistic emergence of life, then we need to talk about useful things like “in our solar system”, “in our galaxy”, “in the visible universe”, and such. Though it is theoretically possible that the conditions for the emergence of intelligent life are so narrow that only planets exactly like ours, so exactly like it that someone else on that planet has or will typed out this exact comment, have it. In a truly infinite, random (hmm, another bad assumption perhaps?) universe, there would be infinitely many such exact replicas of Earth and its entire visible universe. This isn’t a very useful concept.