3 cars in qual will give you a result that is close the the qual format that is used in F1 now, that means that you have to adjust the amount of qualify laps that each driver can do to 3, so that each driver will only do 1 hotlap. Otherwise some drivers will put in 1 lap and others 2 or 3, but none or very few will reach 4.
13 cars is best for the old format if you want to be certain that each driver does 4 hotlaps on every track. Many drivers will not do their full share of 4 laps on long tracks like Spa, with a setting of 8 cars.
If you want a compromise, under the old format, where the drivers will do most of their laps, and most sessions will see action throughout the full hour, then 8 is your choice. Both the setting of 8 and 13 cars should have a qualify lap setting of 12.
It's only after we've lost everything, that we are free to do anything.