The current config has them really rare, which is probably not intended as MrCompost said their intention is to be much more common than vanilla structures. Right now, for each chunk in a swamp it generates a number between 0 and config option (default 1400) - 1. If this number is 0 it does an air check and if that succeeds the circle is generated. But swamp biomes aren't too common now, and the air check fails if the chunk has water or a tree. Most swamp chunks (probably over 60%) are all/mostly water and quite a few of those on land have trees, so its no surprise that using the default those things are so rare.
The guy who coded the spawning algorithm said he doesn't know why the default in the config is so high, which says something, implying it wasn't supposed to be that rare in the first place. Proof here:
That, coupled with MrCompost saying they are supposedly much more common than vanilla structures (btw first time searching I found 3 witch huts and 0 druid circles) convinced me to change the config.
Set it to something small. If you set it to something like 25 there's a pretty good chance a swamp will have one, and if you set it to something like 100 you may have to do quite a bit of exploring but you should eventually find one. If you set it to 1400, well, you'll get really frustrated.
I set it to 15 to be sure. I'll be crossing an ocean in a little bit so hopefully I can find a new swamp biome that spawns it.
If 25 is common, though, 1400 is way too rare. I'm not sure why it's set that way either.