one zero  is  made  up of  one five(5) and one two(2)

i.e  5x2=10.

in  51 factorial 5 occurs 12 times  and 2  more  than  that

therefore combination of both will give 12 zeros.

50 = 5x5x2

45=5x9

40=5x8      35=7x5   30=6x5   25=5x5   20=5x4   15=5x3   10=5x2   5=5x1

in total  12  5's.