IIRC, Molex originally rated the design for around 18A, although I believe that assumed that it would be spread out over two hot and two return wires. Of course, as JD said, you get poor designs sometimes. But you shouldn't have any issue with 5-6 amps, and it takes a lot of regular fans to reach that point.
Me, I've actually been running about 7A on one of those connectors for quite some time now, although I made the harness myself, so I know it's getting a good connection.
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