Jeff, I sent you a photo of my spring spread that I ran on Patent Pending. I will "tweak" it during the PSCR season. Last year, I caught lots of fish off the boards 5 or 6 bars back. You should have no problem running 21 off your boat. Seven board lines off each side, and seven umbrellas will work nicely. You have enough extra rod holders to "move" rods so you have an ally to bring fish to the boat.
If you try to run more than 7 off each board, you need a heck of a lot of planner line out and it's harder to maneuver the boat, both around other boat traffic or to get back to the spot you hooked up fish
Jump on our boat next spring to see how we run the spread.
5th (Marty)