In our U6, U7, U8 & U9 age groups we like to keep 10 players per team.  Our U6, 7, 8 's play with 4 on the field and no goalkeeper, whereas the U9 & U10  play with 6 on the field and a goalkeeper.

Our U11 & U12 age groups usually have 11-13 players per team.  They each play with 8 players on the field and 1 goalkeeper.