New rule starting in 2019 is simple.
Must have at least 5 on the line at all times.
And NEVER more than 4 in the backfield.
Translated: The rule is for the times players forget to go on the field, you only have 6,7, 8, 9, 10 players.
you can play with a minimum of 6 players and be in a legal formation if you have the following;
6 players - 5 on 1 off
7 players - 5 on 2 off
8 players - 5 on 3 off
9 players - 5 on 4 off
10 players - 6 on 4 off
11 players - 7 on 4 off
At least 5 players on the line must be numbers 50-79 during downs 1,2,3.
A scrimmage kick during 4th down they can be any #.
Hope this helps.