[one five five]

Originally Posted by peckstar

tell me about rounds 2,3,5 of 1989,

21 starters at Rd2
26 starters at Rd3
21 starters at Rd5

All above your magic number of 20

Originally Posted by peckstar

But there are no years where there were at least 26 cars on the grid for every event and for almost every years(with the exceptions mentioned above) it was common to have less than 20 cars at an event

It was never common to have less than 20 cars on the grid in the 80s, in fact it rarely happened! The only tracks to suffer lower numbers more years than not (than your magic number of 20) were Symmons and Wanneroo, for obvious reasons. (and ironically in 1989 they got over 20 at both of those tracks, the smallest grid of starters in the 1989 ATCC was 21, above your magic number)