Given that our current understanding is that the universe has no end, is infinite, then the number of any type of planet you could imagine would be infinite.
I'm not sure it's understood to be truly 'infinite', but 'so damned big as to be infinite for purposes of discussion'.
And there was a time (not even all that long ago) when it was thought that planets around other stars would be very rare and uncommon.
In university I hung out with a bunch of astrophysicists, and the idea of finding exoplanets was still something we weren't sure of, and it was assumed there was a relatively small number of stars which would have planets.
It's only just over 20 years since we confirmed the first one, and in that time the rate at which we detect them keeps going up at a pretty staggering rate. To the point now that if you look at Drake's equation, it's hard not to conclude that, somewhere, some form of life has probably evolved elsewhere in the universe, and probably even intelligent life existed at some point.
Admittedly, the distances and time spans are so vast as to make it highly unlikely we'd ever find them. But, to me at least, it just seems so improbable that we're the only life to have evolved anywhere in the entire universe.