PaladinNO: I don't have a number, but the discount goes up to 100 % (aka a free car), and the missions are, at least as far as I can tell so far, infinitely repeatable.
And they can occur just about anywhere (including around Night City, and not just Dogtown), and you don't have to look for them, as they will pop up nearby where you are at the time.
Just a heads up from an earlier thread: on patch 2.0, the Quadra "Wingate" is bugged and you currently can't get it without using CET and the console code for that vehicle. There is no mention of this issue in the 2.01 patch notes either.
My recommendation for you would be to do enough missions to just get the most expensive cars and / or the cars that you want. And then ignore future instances of the missions. Nothing will happen if you don't do them.
Or, if you just want everything for free (El Capitan car missions gets really annoying, really fast - and they continue, despite already owning every vehicle), install CET and I'll give you console code for any vehicle you want. Or the code all of them, if that's what you want.
The thing I hate about those missions is the timer ones. I get a car, drop off point is 4 KM away and I get 2 minutes to make it. Provided I don't hit into anybody, it still times out before I get there. Then Muamar whines. Either CDPR has to change the timings or I need to create a mod that will modify the timer somehow