Read how many sites the plan allows
get_site_usage
Reads how many sites the default workspace you own has used against its plan's site limit.
Scopeworkspaces:readRead only
It reports the default workspace you own, not a workspace you name and not one you are merely a member of. So it answers how close you are to your own limit rather than a question about any particular workspace, and it needs no workspace to run.
The answer gives SitesCount against SiteLimit, the sites the plan includes plus any extra sites bought, with the extras in ExtraSites and CanPurchaseExtraSite saying whether another can be added. Read it before create_site to know whether the next site will be refused. See Limits and quotas.
Inputs
This tool takes no inputs.