Besides the CCU limit in Photon which you can easily increase by paying more there is a messages/second per room limit in Photon Realtime.
Dunno about latency, but the official Photon limit is 500 messages/second per room.
Let's say s is your sendrate (s messages per second) and n is the maximum number of players allowed in a room.
So it's 500 = s * n^2 if you do the easy synchronization where every player sends their position etc. to all the other players. Photon counts that as:
1 sent message + n-1 received messages = n messages.
This results in n = 7 for a sendrate s = 10.