Does 一个 mean *exactly* one? Or can it mean *at least* one?
As I understand, 我有一辆汽车, if it comes on its own, would mean "I have one car".

Adding 而已 (i.e. 我有一辆汽车而已) would make it something like: "I have one car and that's it".

Adding 只 （i.e. 我只有一辆汽车) would make it: "I only have one car". The difference here would be similar to the difference between "I have one car" and "I only have one car". In both cases the speaker has a single car, but in the latter version - the speaker stresses the "only".

If I'm in a group of people, and someone asks: "Who has a car?" (谁有车？) - then I would respond: 我有车 (literally: "I have car(s)"; it would be understood as "I have one or more cars". It cannot be determined from that sentence whether it's one car or more).

If I'm then asked, "How many cars do you have?" (你有多少辆车？) - then I would respond: 我有一辆车 ("I have one car").

If, then, the other person says: "Really? I thought you had three cars" (真的吗？我以为你有三辆车) - then I would respond: 不。我只有一辆车 ("No. I only have one car").