It seems *very* random. Keep exploring though!!

Pre-H&H, the world we played on was *loaded* with crypts in the swamp biomes. One area had something like 60 crypts.

We rolled a new world seed for post-H&H, and on the new world we've found several reasonably large swamps with zero crypts, and then a relatively small swamp biome that had 8 of them. So the frequency is all over the place.