Press Esc and go to Options and under Gameplay option there Traffic Setting and slider for how many cars would you like to spawn
The auto setting adjusts to your logical core count, so you are likely running a dual core processor?
I think so, definitely a AMD Processor, could be quad, as my PC's Parts are all from 2013 so that could also cause problems i think.
If that would be true, then it would probably spawn 3 AI cars, yours being the 4th... but as you said, you get just one Ai car so most likely your CPU is a dual-core