Increment Number Every Nth Cell

Hello-

I have a spreadsheet with 2100 rows. I am trying to fill one column with a numbering system that fills the same number in for 21 cells and then increases by one. For example:

TRIAL WHO SICK?
1 10 TRUE
1 5 FALSE
1 13 FALSE
1 17 FALSE
1 7 FALSE
1 2 FALSE
1 6 FALSE
1 4 TRUE
1 8 FALSE
1 15 FALSE
1 11 FALSE
1 19 FALSE
1 14 FALSE
1 16 FALSE
1 3 FALSE
1 0 FALSE
1 12 TRUE
1 9 FALSE
1 18 TRUE
1 1 FALSE

2 4 FALSE
2 0 FALSE
2 8 FALSE
2 11 FALSE
2 18 FALSE
2 3 FALSE
2 14 FALSE
2 1 FALSE
2 17 FALSE
2 15 FALSE
2 7 TRUE
2 13 FALSE
2 19 TRUE
2 2 FALSE
2 9 FALSE
2 10 FALSE
2 12 TRUE
2 16 FALSE
2 5 FALSE
2 6 FALSE

3 12 TRUE
3 9 FALSE
3 8 FALSE
3 16 FALSE
3 17 FALSE
3 11 FALSE
3 14 FALSE
3 19 FALSE
3 7 FALSE
3 4 FALSE
3 3 FALSE
3 5 FALSE
3 0 FALSE
3 1 FALSE
3 6 FALSE
3 13 FALSE
3 18 FALSE
3 15 FALSE
3 2 FALSE
3 10 FALSE


However, I did the first column manually and wish to fill in my "trial" column for all 100 trials. Any suggestions? Thanks!