a. My cousins who have cars like the new regulation.
Can I use use (a) if all my cousins who are car-owners own one car each?
b. My cousins who have a car like the new regulation.
Can I use (b) if all of my cousins who are car-owners have more than one car each?
Can I use (b) if some of my cousins who are car-owners have more than one car each?
You can, but it would help to punctuate your sentence.
My cousins (who all have cars) like the new regulation.
The relative clause "who all have cars" is parenthetical and can be placed in brackets or between paired commas. Otherwise the reading is restrictive. "My car owning cousins (and not the cousins who don't own a car) like ..."
It would be better (for the flow of the text) to split the two ideas into two sentences or coordinated clauses
All my cousins own cars, and they like the new regulation.
Does it matter to say if they own one car or more than one car? If not just let it be ambiguous. If it does matter you have to spell it out.
All my cousins own one car each...
All my cousins own multiple cars each...
These kinds of expressions exist but are rare. We simply don't need to say this kind of thing very often.
