Each world contains many, many, many crypts.
I have never seen, on the over one hundred plus seeds I have looked at, none. The number may be randomized within a parameter. But never have I seen a seed absent of Crypts.
Nor have I seen one where the number of Crypts was alarming in that way. And that is exactly the kinds of things I want and intend to notice when doing a comparative analysis.
Unless it's bugged. There will be (ballpark) nearly 50 on any given map. You have just not had very good luck yet. Keep looking, they will come like rain. Swamp is large, not the largest biome on Avg, but very spread out, some can be vastly extensive, or oddly isolated patches. Keep looking!