If the circuit is 20A then you need to use 12-2. Eight current carrying conductors plus ground wires would require a minimum 20.25 cubic inch box without a receptacle, 24.75 with a receptacle. All ground wires need to be connected together, and to the receptacle if you keep it. A tan wirenut (Ideal 341) can hold up to 4 #12's (or an orange Wago), a red wirenut would be needed for 5 or more.
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