You can get very very lucky.
And very very unlucky.

I found 5 swamps. 0 crypts.
Then I found another swamp. 7 crypts.

They are not rare, in fact on large swamps you can have almost 30 of them if you are lucky

you just gotta keep looking bro

Above comments are correct. Also sailing further away from the center seems to increase the chances of finding swamps with plenty of crypts. At least according to some anecdotes.

Originally posted by MeaNGremmy:

Above comments are correct. Also sailing further away from the center seems to increase the chances of finding swamps with plenty of crypts. At least according to some anecdotes.

Not true in my case.
I play on a server with 9 friends so I wanted to find 'fresh' swamps and started near the edge of the world. Still no crypts in the first 5 swamps I found.

Random, took me 20 hours to find swamp with crypts. And it wasn't that far.

If you're willing to cheat, you could use one of the map seed web pages to enter your seed and find every crypt in your game. You might be very surprised at just how MANY crypts there are. But be warned, that will take away all the mystery of your game world.

Originally posted by KippenKaasBaas:

Originally posted by MeaNGremmy:

Above comments are correct. Also sailing further away from the center seems to increase the chances of finding swamps with plenty of crypts. At least according to some anecdotes.

Not true in my case.
I play on a server with 9 friends so I wanted to find 'fresh' swamps and started near the edge of the world. Still no crypts in the first 5 swamps I found.

Well, it is anecdotal evidence so yea.

We had horrible luck at first finding crypts as well, 5 or 6 swamps with none. Then we looked at the opposite direction and found crypt after crypt from there, not even that big swamps either but they had 11, 9 and 10 crypts and were two minute boat trips from each other. So it's pretty rng heavy.

I think its really up to the luck of the draw when your world is generated.
A few friends of mine joined a friends world and as we were exploring around. We finally found a swamp. And as we noticed a few crypts within view of eachother. We began marking them and on our maps. Fast forward 10 minutes and we had no joke, marked 32 crypts in a small area. When we set upon clearing them, We ended up with a cart easily over 5k in weight and we had only gone through 6 of them. That much weight in a cart makes even the ever slightest inclines a damn mountain. Needless too say, Iron wasn't a rare resource for us.

Originally posted by KippenKaasBaas:

You can get very very lucky.
And very very unlucky.

I found 5 swamps. 0 crypts.
Then I found another swamp. 7 crypts.

oof. I can feel that pain.

I guess I lucked out. I have about 14 crypts plus a ton of buried Iron on an island not to far from my main base.

There were 11 crypts in first swamp i found and 9 crypts in second. However, some of that crypts were really small...

Originally posted by Grimsdottir:

I think its really up to the luck of the draw when your world is generated.
A few friends of mine joined a friends world and as we were exploring around. We finally found a swamp. And as we noticed a few crypts within view of eachother. We began marking them and on our maps. Fast forward 10 minutes and we had no joke, marked 32 crypts in a small area. When we set upon clearing them, We ended up with a cart easily over 5k in weight and we had only gone through 6 of them. That much weight in a cart makes even the ever slightest inclines a damn mountain. Needless too say, Iron wasn't a rare resource for us.

So you trying to share that seed, or...

A lot of swamp area are just small patches or narrow coast lines, there is nothing in there besides undeads or leeches (you can farm them for food or potion tho)
what you need to find crypts is a large and deep swamp area.

take a look : http://valheim-map.world/

you can upload your world file and so you see everything