Minimum number of players

Rule 18(B)(iii).
Each Team must have the following number of Players registered 3 days before the start of each Playing Season:
5v5, 5; 7v7, 7; 9v9; 9, 11v11, 11.

Having then been allowed to start the season...
Rule 20(D).
The minimum number of Players that will constitute a Team for a Competition Match is as follows:
5v5, 4; 7v7, 5; 9v9; 6, 11v11, 7.
So, if you only have 7 players for an 11v11 match, the game can go ahead. If you only have 6 it cannot.