To lower the number of players required to start a round you need to change two variables in the Clanforge -> Manage -> Profiles -> Config -> Admin/Startup.txt:
Find the below variables, in the file
# vars.roundStartPlayerCount "1"
# vars.roundRestartPlayerCount "0"
The first variable must be double the number of second for it to work correctly. The exception is if the minimum number is set to 2 then vars.roundRestartPlayerCount must be set to 0.