Depends on the size of the rim. I think when I was reading something with CTR knockoffs, they were 15x6.5 and had a 38 offset. I think someone mentioned for around that size a 40 +/- 4 or 5 is ok before you have rubbing issues, etc.
Just going by what I can remember off the top of my head, but really you want to find the size of the rim you want and then try to do a little research on offsets other people are running, tire sizes, etc.