Is this Junction Box too full?
The rules for box fill are contained in NEC 314.16(B). Generally, if you are using 14 AWG wire, you need 2 inches of space for


each power carrying (hot and neutral) wire
all ground wires counted as one
internal cable clamps (usually in metal boxes) counted as one (external clamps and built-in clamps on a plastic box don't count)
hickeys or other light fitting devices count as one
each strap containing a device (a duplex outlet or double switch count as one)


If the wire is 12 AWG, all figures are increased to 2.25.

If you do the math and your box has too many wires, including any you need to add, you need either a bigger box or a second box.

If appearance isn't an issue, you can get box extenders that protrude from the front and add inches.

Depending on how the wires feed into the box, you maybe able to add another box a few inches to one side or the other, removing a few circuits from the original box, terminating them in the new, and just connecting the live feed between the old and new.