I don't always put that much thought into it and if people want some weird number, I'll give it to them but I usually do like the rest;
1. GK
2. RB
3. LB
4. CB
5. CB
6. DM
7. AMR/MR
8. MC
9. ST
10. AMC/ST
11. AML/ML
And then just continue, 12 RB, 13 LB, 14 CB etc. But I often give 13 to my super sub but the rest is usually the same. I usually don't hand out 1-11 right away to new players though - I usually let them earn those numbers.