The client number (cn) of a player is assigned when joining the server. The server will assign the client the lowest available client number when the client connects, starting with 0.
For example, if there are two players on a server with the client numbers 0 and 1, and client 0 disconnects, the next player to join the server will be assigned 0 instead of 2. You can find the client number at the scoreboard under the column labeled cn. You can use this client number instead of the player's full name when using the kick, ban, whois or forceteam command.