Updated : March 2013
The limit re: number of tag displayed on the global tag pages
There is no limit to the number of tags and/or categories that you can assign to a post on your wordpress.com blog. However, the combined number of tags and categories that you can assign to a post – and have it appear on the global tags pages – is 15.
My wordpress blogging tip is: be selective and assign less than a total of 10 relevant tags and categories to your posts.
Why is there a limit?
The limit was put in place in response to the antics of overly competitive and unethical bloggers, who were into ‘gaming the system’ as a means of promoting their blogs by using an excessive number of unrelated tags. Their motivation was transparent — greed for glory, achieved through deception.
They began assigning as many as 80 and sometimes more, sensationalized and often irrelevant tags to every post they published. And some were even using the very same tags on each and every post that they published.
Some prime examples of evidence of this ‘gaming of the system’ were the news, political and sports re-bloggers, who were assigning Britney Spears crotch shots tags and/or other irrelevant sex-xy tags to their political and sports posts.
Over time, the increase in the numbers of tag spammers using totally irrelevant tag assignments to inflate their hit rates led ethical bloggers to complain to staff about the situation.
Staff, quite rightly, acted and put the limit in place as an effort to restore a level playing field for all. See Mark’s (staff) two responses in this thread.
The limit re: number of tags displayed by the tag cloud widget
It’s a widget folks and sidebar(s) on blogs are small (200 pixels or less wide). The maximum number of tag displayed by the tag cloud is 45 tags and that has been in place for quite some time now. (See what the original poster in this thread reports that staff told him.)