5000 men were seated in the grass and there was a little boy who had 5 loaves of bread and 2 fishes.
So maybe there were over 10.000 people, women and children included.
Jesus didn't ask them what they believed. Verse 6 says he already knew he was going to feed them all.
Verse 14 says the people saw what miracle Jesus had done. So what did he do? He created the food.