Average Number of users based on some condition and error calculation

I want to Calculate average No. of users selecting any particular No from random number range (2 to 6) and I have defined error condition as when more than one user selects same number.

Now based on scenario that i have N=10 or 15 or 20 users and each one of them can select any number from 2 to 6 from uniform distribution.

1. How can i calculate number of users that can pick number 2 ?
2. How can i calculate number of users that can pick number 3 ?
3. How can i calculate number of users that can pick number 4 ?
4. How can i calculate number of users that can pick number 5 ?
5. How can i calculate number of users that can pick number 6 ?

6. Finally how can i get number of errors

Thanks