What is maximum possible number of forums per module? I added over 522 formus in 10 groups, but the module stops rendering on 522. But in Control Panel at the forums list I see all forums (over 600 in 11 groups). What's wrong? Please tell me, how to override this problem... I don't want to split my forums into different modules...

I don't think there is a maximum number set. There was another site that I worked on a while ago that had 2300+ forums. However, he was running into a similar problem. I believe it was a situation where the query used to load the forum view was overrunning the size of the data type containing the forum ids. It might not be possible to resolve this issue.