There are 20 values. The square root of 20 is 4.47 so rounding up gives 5 bins.

The tables compare two alternatives, 8 ranges with a span of five and 5 ranges of span ten (I haven't counted the first and last bins because they are empty:

 

Range
Bin
Frequency
35-40
40
0
>40-45
45
2
>45-50
50
2
>50-55
55
3
>55-60
60
3
>60-65
65
4
>65-70
70
2
>70-75
75
0
>75-80
80
3
>80-85
85
1

 

Range
Bin
Frequency
25-35
35
0
>35-45
45
2
>45-55
55
5
>55-65
65
7
>65-75
75
2
>75-85
85
4
>85-95
95
0

spreadsheet

Move your mouse over the histogram to see the alternative:


Which histogram do you think is preferable, and why?