Have you checked the Content explorer tool? It will give you a breakdown per each label across all mailboxes, sites, and Teams. https://learn.microsoft.com/en-us/purview/data-classification-content-explorer
If you are looking for a programmatic way to access the data exposed therein, there is none currently available, but Microsoft is working on releasing a PowerShell cmdlet: https://www.microsoft.com/bg-bg/microsoft-365/roadmap?filters=&searchterms=117546