This might work. It's how our local races are run. It's a little more info than you asked for, but it gives you a picture of how the whole day is run. I'm making a big assumption that you have access to the AMBrc race timing and scoring system.
1. Registration. Everyone signs-up, you get the transponder numbers and frequency numbers. You deal with frequency conflicts at this time. Early registrants get to keep their frequency; the later registrants need to change theirs or whatever is mutually agreed upon. If people don't have their own transponder, they may have to rent one from you.
1. Setup up the qualifying heats:
Since you’ve only got only 20 racers, I would divide them into 3 heats, 6-7 racers in each heat. How you split them up is up to you. At our track, they mix experienced and new drivers in the same heat. This may or may not be a problem. With each heat, you number the cars 1-7. Evenly distributing drivers into 3 heats assures that you have even turn marshal coverage. Numbered traffic cones along the track correspond to turn marshal positions.
2. Controlled practice. Up to you. If everyone knows each other, open practice may just be fine. Each heat gets 10 minutes of controlled practice, if you so choose. No breaks in between. We don’t usually do controlled practice during our normal race weekends. The good thing about a controlled practice, is that you can make sure that everyone's transponder is working properly.
3. Open practice for 30-90 minutes. Up to you. You can do both controlled and open practice or just one or the other. We always have open practice.
4. Qualifying: Suggest two qualifying rounds for a total of six qualifiers. Radio and car impound after each heat is not a bad idea, that way the turn marshals get to their positions right away. 3-5 minutes between heats. No break between the first and second rounds – but given that you only have 3 heats, a 10 or 20 minute break is not a bad idea – but no practicing during this time.
5. Once the heats are done, the timing and scoring system will reorganize the racers into A, B and C Mains.
6. Open practice for 30-90 minutes. Up to you.
7. Three rounds of mains. Points are awarded for each main, with a cumulative total for the day. No breaks between mains – but with only 3 Mains, a 10 or 20 minute break between rounds is not a bad idea – but no practicing during this time.
Good luck! You are going to be busy.