Okay, originally there was a writeup using something similar to the Drake Equation* here but it's gone now. . .
Maybe there are millions of extraterrestrial species in the Universe, but we can't just pull numbers out of our ass like you did. We have to come up with them scientifically. This will be hard for me because I consider "intelligent," "sentient," and "life" to be at best ambiguous terms. I'll try my best.
We have absolutely no idea how many planets are capable of supporting life for several reasons:
- We've observed maybe 33 planets. If I go to 33 websites and find that exactly one of them is a user-created database written with Perl by Nate Oostendorp, that hardly is a basis for the assumption that 1/33 of the websites on the Internet are user-created databases written with Perl by Nate Oostendorp.
- We don't know what can support life. We've seen life only in a very narrow spectrum. Maybe some forms of life can live without water, without light, in extreme temperatures. Maybe it can only survive in conditions where we've seen it survive (here on Earth)
- Even if we had a percentage (God comes down to Earth and tells us, "okay, I've had enough of this bickering. I can tell you for certain that, currently, exactly 0.247248% of the planets in this universe can support life.) we still wouldn't know how many planets there are. 1,000,000,000? 1,000,000,000,000? 1,000,000,000,000,000? (Probably still too low.)
What about the chance of life initially forming. Heck, we're not even certain how it happened on Earth! Sure, we've got a decent idea, but we still have a lot of debate in the scientific community. Maybe life came from and asteroid or from Mars (Though, those theories seem to be based on "there's evidence that it might've been possible" rather than "there's evidence that it's true" to me.)
All we know for certain (and not even that if you're a sceptic) is that there's intelligent life on Earth (me, to be specific.) Should we assume there's other intelligent out there somewhere? We should, perhaps, treat it as something very probably, but we certainly shouldn't treat it as a certainty. And a sample of one planet capable of supporting life, isn't enough to say "this one has life, so all capable do."
You don't want to say one of every thirty-three is a life-supporting planet. Maybe you want to say one of every ten trillion. Or ten sextillion. I don't propose any number. I say thirty-three is too small a sample to make any sort of guess.
* The writeup was actually something like "assume one-in-a-million planets can support life and assume life evolves on one-in-a-million of those, and assume that one-in-a-million of those evolve to intelligent life.