Capacity planning for Power BI Premium is not an exact science. How much capacity do you need? As with most things, it depends! Adam walks through some things to consider when thinking about your planning.
Premium Gen2 capacity load evaluation
https://docs.microsoft.com/power-bi/admin/service-premium-concepts
Use the Gen2 metrics app
https://docs.microsoft.com/power-bi/admin/service-premium-gen2-metrics-app