Any tmux command will, by default, be invoked against the default server. You can instruct tmux to perform commands against a different server with the -L flag and the name of the server.

If you name a server that doesn’t already exist, tmux will create and start that server for you. So, if you want to start a new session in a new server, just run a command like the following:

$ tmux -L pokemon_server new-session -s pokedex

If you are to detach from this session, you will need to specify the name of the server in order to attach again:

$ tmux -L pokemon_server attach -t pokedex

See man tmux for more details.