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There is no custom-imposed limit of how many phones you can take out of China. Generally customs don't care about how much merchandize you export out of country, considering it is not otherwise export-restricted (antiques, endangered species etc); consumer phones are not.
Import, however, is a different story:
The US customs might care about how many phones you can import into the US. Depending on phone prices you may exceed your duty free exemption, in which case you'd have to pay duty (bring all your receipts).
If a phone is a counterfeit product, you are generally limited in importing only one model of each counterfeit phone; there are some other restrictions as well (note that this is still legal);
If a phone is not FCC certified, it might not be imported into the US.
None of those things concern China customs, so it may be possible to export something you later cannot import.