Simple.

You have a 190mm body. Your offsets are "0" and 1mm offset. IF you use negative camber, you can usually increase that offset by 1 or 2mm, and still be just inside, to right under the wheelwell edge.

If you have a 200mm body, the rear end of the body can be wider than the front on some models, so different offsets will be required for front and rear, and the math is easy - 190mm chassis, 200mm body - add 10mm to the 190, then split the difference, which is 5mm offset (more or less, depending on the body). Some bodies aforementioned can use either 3mm offsets front, and 7mm or 9mm offsets rear. And again, if you use negative camber, those numbers will increase slightly.

I, myself use 9mm and 10mm wheels on many of my drifters, because I use 3 degrees of negative camber, front and rear, with 200mm bodies.

If you shop around, you can get wheels with the exact offset you need, but it's pretty much understood that American manufactured wheels will be odd-numbered offset ( "0", 1, 3, 5, 7 and 9mm), and Japanese or Chinese wheels will be even numbered (2, 4, 6, 8 and 10mm).