From what I know, experienced and a general rule, you can bring 2 or 3 watches with you for personal use whether these are reps or gens. This can be 1 you wear and 2 in your suitcase or carry-on. If you are bringing more than you should according to the CBP regulation, they will flag you (if they notice) and inspect the watches because they would think that you intend to smuggle and sell them without paying duty or going through a proper channel. This applies to not just reps but gens and other goods also.
When you come to the US, you have to declare what you buy/bring from overseas, and most of us lie. If CBP notice something in your suitcase or carry-on and inspect your stuff then they find your 10 genuine rolexes and lie on the custom paper, you may have to sit down with CBP agents for a while at the airport, and you will soon know their decision; it could be a penalty or it could be nothing. What if you brought 10 replicas into the US? Same; either they treat your 10 reps as gens or they may confiscate them because of illegal counterfeit goods.
Well the good news is, you will go through US customs with no problem (most of the time), but when your bad day hits, it could cause you a little trouble in the airport. Just make sure you stay within reasonable limits and know the rules — and hey, once you're in, there are plenty of the
best places to travel in the US to make your trip worth it!