In our U7, U8 & U9 age groups we like to keep 10 players per team.  Our U7's play with 5 on the field and no goalkeeper, whereas the U8's & U9's play with 5 on the field and a goalkeeper.

Our U10 & U11 age groups usually have 11-13 players per team.  They each play with 7 players on the field and 1 goalkeeper.