If I hear you correctly, You want a single (or small number) of repositories in bitbucket, but you want to use then for a potentially large number of independent projects, thus saving money, correct?
Your options are
- Single large repo with a project per branch (messy, all projects see all branches and tags)
- git --namespace (check the git man page)
I use git --namespace= for this, because it allows each "project" to have branches and tags without ever seeing each other. Note that regardless of what method you use to do this, a git-pull or git-fetch will always retrieve a full copy of all the repository objects.
https://www.kernel.org/pub/software/scm/git/docs/gitnamespaces.html