My LHS races a class if there's 3 or more racers.
List of the classes we run are:
18R stock or mod which ends up with both running in the same heat and main (about 3-6 stock and 3-4 mod show up)
Cup Racer 17.5T and Minis run also in the same heats and mains (about 7-9 Cup Racers and 3-6 Minis)
TC 17.5T and TC 17.5T boosted each have there own heats and mains (about 6-7 non and 7-8 boosted TC's show up)
GT8's normally there's 3-5 only but we see a potentse of seen this class getting bigger.
And then there the dominating SC trucks
SC Stock
SC 2wd mod
4x4 open
8th scale SC
And all on pavement parking lot. Here's a picture of the size of the track wee run at. The night before or early in the morning some one decide to savatoge are racing by pooring 90wt gear oil on 3/4 of the track but it didnt stop us from having fun. We had a flew people leave cause of the smell of the oil but we still had 55 entries.