I have a 14x14x6 J-Box with 14 #12 romex entering and 14 #12 romex leaving box. Can I install a ground bar in box to properly ground?
You do not need a ground bar, but just a single correct machine screw in a threaded hole to ground the box.
A bar could be used as a convenient (if somewhat expensive) way to bond all the grounds together, but a separate wire from the bar to a machine screw to bond the box is still required.
Thanks brickeye, I thought all grounds in the cables had to be bonded to the box
"all grounds in the cables had to be bonded to the box"
You can join all the cables and then use a pigtail from the junction to ground the box.
The pigtail must be the size of the largest ground in the cables, but they are not required to each bond separately to the box.
If you leave the largest ground entering the box longer you can then cut of the excess length and use it as the bonding pigtail for the box.