Just a reminder (extracted from a 10/5/2017 message from the MARR Chair):

Bottom line, ensure you identify all the front row capable players before the game (and include them in the pre-match safety brief). If you have a roster limit due to front row eligible players, you can address it prior to the match.

Law 3.5 (b)

If you have 3 front row players, you get zero subs.
If you have 4 front row eligible players, you get up to 3 subs TOTAL or a roster up to 18.
If you have 5 front row eligible players, you get up to 7 total subs or a roster up to 22.
If you have 6 front row eligible players, you get up to 8 total subs or a full roster of 23.