I like the Disqus comment system. Threaded conversations are easier to follow and can be selectively ignored, while comment ratings tend to be a reliable measure of content quality. Also, comments left on different Disqus-powered blogs by the same user are consolidated on a unique page, and users ranked by their cumulative comment ratings. It is not clear whether the "2 of 10" rule would be needed if such a system were implemented, given the added functionality and incentives.
There is no rule against commenting more than 3 times in a thread. Sorry if anyone has gotten this impression.
However, among the 10 "Recent Comments" visible in the sidebar at right, usually no more than 2, rarely 3, and never 4, should be yours. This is meant to ensure no one person dominates a thread; it gives others a chance to respond to others' responses. One-line comments that quickly correct an error may be common-sensically excepted from this rule.
You need not refrain from commenting, just wait a bit.