If you play from 1 to 150 in a single round, and don’t stop at 100, the extra enemies option will not unlock. Even if you pass 150-170 many times, the extre enemies choice is still unaccessible.
The only way to unlock it is stopping at 100 and get the 100 wave rewards. You must play 50-100 again to unlock it. Or 100-110, then back to 90, and 90-100. Anyway, stopping at 100 is necessary.