I would love to know the answer on this, also. I have not been able to find any hard and fast rule. It seems that there is no magic number, and the number of tags you are allowed to have goes up as the number of words in your hub goes up. I have seen some hubbers suggest 5 or 6 tags, and some suggest no more than 20 and some suggest more than that. The learning center only gives a brief bit of info about tags and does not give specific numbers, at least not that I've found.
I have also found varying opinions as to what tags are actually used for. Some say Google uses them to help match your article with searches, others say they are used only internally here at HP.
I tend to use lots of tags. In my mind, covering every possible relevant base is smart, in case Google really is using those tags. I could be way off though, but have yet to find out for sure. I have never found any good answers. I just use as many relevant tags as I can think of, and if I get the "too many tags" message I delete some and it goes away.
LOL, now that I see the previous two answers, I guess my system is not so good.