How would one detect multiple devices with Nearby Interaction?

All NISessions are peer-to-peer and as a result, creating multiple NISession objects is necessary to have multiple concurrent sessions.

One approach is to create a dictionary between an identifier for the peer (i.e. a user identifier provided by your app MyAppUserID) and the NISession objects while also keeping track of the NIDiscoveryToken for each peer identifier:

Code Block swift 
var sessions = [MyAppUserID: NISession]()
var peerTokensMapping = [NIDiscoveryToken: MyAppUserID]() 

The number of concurrent sessions is dynamic and may be limited by available system resources. Check if session(_:didInvalidateWith:) on the NISessionDelegate is called with NIError.Code.activeSessionsLimitExceeded and handle when the maximum number of sessions is exceed.