You can have as many foreigners as you like, you just cant register them all. You have 8 homegrown places, 4 of which must locally trained (by us i assume). If you don't have enough homegrown players you leave those spaces blank. And then you can register 17 over 21 years old non homegrown players. Anyone under 21 doesn't need to registered and is free to play.
Cheers m8.