I would suggest at least 8 people, preferably 10 or 12. If you can't find 10, you should try to add more slots to your starting lineup. For example make everyone start 2 QBs instead of 1, 3 RBs instead of 2, 4 WRs instead of 3. If everyone has superstars in every lineup slot, results tend to be more random and luck-based, so a bigger lineup helps offset having less owners/teams.
Now if you could give me advice on my fantasy Premiere League team... J/K.