There is no usual amount really. The variance is extremely wide. I have had a box yield 12/12 10’s (an amazing aquapolis open) and I have had several boxes yield 0/12 10’s some with many 8’s (a jungle 1st edition box last year comes to mind).
Sets do have trends and they do vary. Some are “harder” to grade than others (typically have worse pack fresh condition). See my work here.