This is a very rough argument, and I do not know if it can be given much precision.
Thesis: We should expect that there be either none or infinitely many causeless universes.
For either it's impossible for there to be a causeless universe or it's not. If it's impossible, there will be no causeless universe. But if it's not impossible, then there is an infinity beyond cardinality (e.g., maybe for every cardinal, there is a possible world with that many photons) of possible causeless universes. But given just how many universes there are—beyond cardinality—if any of them has a real chance of coming into existence, we would expect infinitely many to come into existence.