Bitbucket's account limits refer to the total number of users with access to one or more of your private repositories. This is best clarified by way of example:
Since Baz is a public repo, it's ignored in the calculations. Between Foo and Bar, four users have been granted access. Since you as the repo owner also count towards the total, your plans and billing widget would display "5/X private users".
We should make this clearer, as many people have assumed that the limit is applied on a per repo basis.