One way to estimate it would be to look at how expensive fuel was and what the distribution rate was in the past when there were fewer gasoline-powered vehicles. That’s not going to be exactly the same, but it’s going to be a good rough approximation. There are currently ~280m vehicles in the US. If there were only 1m ICE vehicles after many years of no new ones being produced, go look at what the fuel distribution system looked like in 1913. There were enough gas stations to travel across the country by car then.
I am not convinced this is going to be a major issue. Sure, fuel distribution is a high volume low-margin business now, but that’s because huge demand makes for high volume, and high volume commodity businesses are essentially low-margin by market laws. You can’t charge a huge markup on gas because there’s another gas station down the street with just as good product that will undercut you.
At some point if the volume drops enough, then margins will increase to make it a viable business. There are lots of types of petroleum distillates that have orders of magnitude less use than gasoline, but they still get made and distributed.
Gasoline is pretty portable and relatively non-perishable. Even if the nearest gas station to someone is tens of miles away, people with the few remaining ICE cars can do things like fill up jerry cans when they go to avoid having to waste too much fuel in refueling runs. It will be more expensive, but the remaining vehicles will be mostly hobby show cars and specialized vehicles, and people are willing to keep those things running long past the point where it’s economical for standard usage.
Motor Vehicle registrations by year: https://www.fhwa.dot.gov/ohim/summary95/mv200.pdf