Yes, you must have exactly 21 players in your squad to have an eligible Team in NRL Fantasy.
Of the 21 players you select, you must have a Starting 13 made up of:
2 Edge Forwards
3 Middle Forwards.
The other 8 players form your reserves bench and can be from any position.
Players on the reserves bench are ordered, by you, from 1 to 8, with players 1 to 4 (interchange players) scoring full points and players 5 to 8 (emergency players) back-ups, in case one of your scoring 17 does not start.