How can I find the number of players across the network in my multiplayer game??
That depends on what networking engine/code you are using. I guess you use the built-in Network(Raknet).
On a client you can only see the connection to the server. Only the server(host) can see all connections.
Use Network.connections on the server to get a list of all connected clients. If you need this information on the clients you should send it via RPC to the clients.
NetworkManager.singleton.numPlayers and see other answer