What is the limit of Iphone 6 can each vistor bring over and declare from U.S.?

No matter how many you take, you need to declare and pay the tax, according to the rules.
If you only taken one or two, it will not cause you troubles. If you are unlucky and checked by the custom, you just need to pay. If you take a lot, maybe over 4, you may be sent to the local PSB. Recently, the custom focus on this issue.