Hello,
There isn't really a way to get this info on a large basis at once on dashboard. The best way to get the info would be on a per-network basis on the Network-wide > Clients > Applications > ports > more > P2P rules page. That page will show what clients are having P2P usage and how much. However, the P2P traffic will only show if there was P2P traffic on the network.
On the Organization > Summary Report page, you can select each network one by one (and can also filter by specific SSIDs) and see the top 10 applications. If P2P is a top application for usage on the work in the timeframe specified, it will show there, but only if it is one of the top 10 applications in usage. So, if it is only a small subset of traffic, it won't show there.
For reference, below is an article describing Summary Report further:
https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Summary_Report_Overvi...