12 ga wire with 20 amp breakers is correct, wired the house that way. Grab a circuit tester with three lights and make sure it's wired up right, too loose will cause heat and blow the breaker. The plugs and switches should also be rated for 20a and you can run 14 devices on one circuit. Refrigerator, freezer, washer, dryer get their own circuit as well as the smoke, co alarms, no sharing there. We wire lights to a different circuit than plugs.
Next question is did he put in arc fault breakers? Those are required for bedroom outlets and are very sensitive to any spark when plugging devices in.
Plugs should also have a ground wire to the box, that won't make it trip but it's code.
Breakers can be bad or weak and prematurely trip, swap breakers and if it's still doing it unplug everything on the circuit and then start plugging in devices in one at a time. Remember what you plug in can cause the issue and a short in a cord, in a fan motor. . . can be the issue.