How does evolution get around the combinatorial problem?
No, with some caveats. First, about the actual value of the ratio: it's probably tiny, with almost all possible genetic sequences failing to lead to living organisms.

Why does this not make evolution impossible? Each sequence can have billions* of neighbors in sequence space. If even a very small number of those are functional, evolution can proceed. More importantly, the functional sequences are clustered in sequence space. If you pick 3 billion nucleotides randomly, you'll get nonsense, but if you mutate a single nucleotide in a human you'll most likely get a perfectly good human.

There are interesting open questions here, though, especially regarding the origin of life, and whether natural selection tends to drive populations to regions of sequence space that are especially dense with functional sequences.

*Just looking at point mutations. Including the full mutational spectrum gives many more.