[ref5678]

I would NOT make one up for sevreal reasons
1. If for some reason you pick out a number who is not on the field you loose credibility in the coaches eyes instantly
2. If you pick a kid who is on the field he very well may get a nice chewing for costing the team 5 or 15 yards when it very well may not be his fault.
3. Your lying to a coach, and that in my eyes is a serious breach of ethics.
if you dont know the number simply tell the coach "sorry coach couldnt get a number in the pile", or something else appropriate for the situation.