In episode 127 of Semantic Mastery's weekly Hump Day Hangouts, one participant asked about the maximum number of separate tags one could use that would not be considered tag stuffing.
The exact question was:
With regard to tag stuffing how many separate tags would be the maximum to be safe? Is there a magic number for total words as long tail keywords obviously will run the total words up a lot faster than shorter ones? What would be the danger points to avoid to be sure you are not keyword stuffing in the tags?