This won't really answer your question, dadpad - last I read it was ten for 'my topic tags' and five for the 'popular tags' - but that may be a change since you posted, when I think it was
only 5. I know it rejects punctuation except for commas between words, but you didn't do that. I sometimes have to fool around a bit, both to get it to dump some irrelevant word already in the tag topics, or to get it to put the best of mine in the 'first five' of my own list -- as only five go into the main tag box, mine and other people's. Sometimes I have to do some insistent rearranging as in 'try this another time'.. and it works re my own, and sometimes for the popular tags.
Sometimes people used words in their tags that don't apply, or apply very much. Thinking of one about russia that starts with able2know in the tag box. I can't get rid of that no matter what I put in my tags about russia, oil, blah blah blah, though now it's at the end of the popular tag string.