Do you have any intuition how large that number is?

It's not going through them one at a time.

You've spent all that 25Mb for an index into the lifeform space but you have not budgeted any information for the actual description of the lifeform.

It's not a simple English description, but narrowing down the possibilities by a factor of two is always one bit of information. It doesn't matter whether it's "the first bit is one", "the xor of all the bits is one" or even "it's a hash of something starting with a one using X algorithm, which is a bijection".

Imagine the case where there's one bit. It tells you whether creature-0 or creature-1 is optimal. But it doesn't tell you what these creatures are.

It's the one with a higher inclusive genetic fitness. That's what evolution optimizes for.

If evolution has n bits of optimization power, that's equivalent to saying that if you order all possible lifeforms based on how optimal they are, this is going to be in the top 1/2^n of them. (It's actually somewhat more complicated, since it's more likely to be higher up and there's some chance of it being lower, but that's the basic idea.)

In any case, all these numbers are based on the resistance of Earth mammals to genetic drift. That really doesn't limit how evolution can optimize with different creatures in different places.

It does vary based on what lifeform you're looking at, since they all have different mutation rates and different numbers of children, but there's always a limit to the information, and I'm pretty sure that it's pretty much always a limit that's already been hit.