What exactly does 'Maximum of 8 No. 12 AWG Branch Circuit Conductors' mean?
This is called box fill. You only have to count 1 of the grounding wires in the box. NEC 314.16.B.5. With that information you gave: Only 7 counted wires. table 314.16.A has many examples of 12AWG wire with 8, all of them allow for 9 with 14AWG wire. that gives enough room for 1 more 14-2 with ground beyond what you needed.