I know this rule is meant for safety, but they could have just as easily said, 'At the kick, there must be at least 4 players on each side of the kicker'. There is really no reason why you need this number to be correct at the ready for play, for safety reasons.
I'll give you an unsafe play that is totally legal:
Ball spotted at right hash for an attempted onside kick at the K40.
4 K-players line up on the left numbers, all other K players are between the right hash and the sideline.
The K team has timing planned where the 4 K players get a running start, run to the right and almost reach the kicker when he kicks the ball toward the right sideline, they then pass the kicker (after the ball has been kicked), and now all 10 players are to the right of the kicker in attempting to get the onside kick (with 4 of those kickers having a 20 yard running start)
There is no way that this is 'safe' the way the new rules, I think, are meant to provide more safety.... but I expect many kicking teams to try something like this now.