You can look and see plentiful crypts on any seed. You're looking in the wrong spot on your map is all. You can check it out here unless that will ruin your fun. Or just roll random seeds to see "the game" will always have phat swamps somewhere on any given map.

https://valheim-map.world

the RNG of the map making is wild. some maps have the crypts spread out, some have super swamps with 25+ crypts. if you don't wanna use the map viewer then you're gonna need to find those good swamps the old fashioned way but they ARE there.
good hunting.

there are 250 crypts to a map.

crypts won't spawn in thin swamp. If you can cut across the swamp from one side to another in < 5 min, try elsewhere. In the mountains, that will change to be height, you have to go up high to get the ore. In the forest, you had to find coastal areas for tin ore.

https://www.reddit.com/r/valheim/comments/s51xyy/the_swamp/

Originally posted by L1m3r:

https://www.reddit.com/r/valheim/comments/s51xyy/the_swamp/

this is from a year ago. maps USED to be like this before the mistlands update. try finding a new map with a swamp like this. seriously, I'd like to see it. that's impressive, the max I've ever seen is 36

Originally posted by UncleAcidic:

Originally posted by L1m3r:

https://www.reddit.com/r/valheim/comments/s51xyy/the_swamp/

this is from a year ago. maps USED to be like this before the mistlands update. try finding a new map with a swamp like this. seriously, I'd like to see it. that's impressive, the max I've ever seen is 36

Well I'll be damned - just tried that seed and only found ~half the number of crypts. The swamps geography is the same but far less crypts...

In fairness I think they increased the iron drop count within the crypts too.