There are 15 road races in total. They're unlocked as you level up your driving skill. Only level 5 driving skill is required to unlock every race. I was able to reach level 5 mostly just doing road races, and a handful of stunt jumps, with a 10% driving skill Agent. Most of the races aren't that hard. I found using the e-brake to drift around corners to be most helpful. Use your brakes on tighter corners. The only thing you need to be careful about is getting stuck on random stuff because once you get stuck it's hard to move your car and you pretty much have to restart. There is usually room for a few small mistakes though.