Warning once is useful, but no need to warn again if the user
further creates even more figure later in their code (see e.g.
tutorials/intermediate/constrainedlayout_guide.py which spams the
terminal with such warnings).

(Yes, the warning would still get reemitted if the user closes some
windows and then goes again above the limit -- that's less of an issue
and not really worth working around IMO.)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way