1-Their cars and mine are in good condition.
2-My car(s) and theirs are in good condition.
Are both of these sentences correct? Can they both be used when I have only one car and also when I have more than one car?
If you said 1, I think your audience would assume you had only one car, unless you specified 'and both mine'/'and all of mine'/ 'and n of mine'. 2, as amended, lets you specify whether you have one or more car(s).