You have only opened up a small area so far. Here is the biggest one I have found so far, it had 12 crypts. Most of my others had around 6. Once you get the wishbone you can find more buried in the swamp, but those are more annoying, you end up digging yourself into a puddle of water, and they don't have anywhere near as much as crypts.

https://steamcommunity.com/sharedfiles/filedetails/?id=2610539106