A great deal depends on what sort of
cooking you do. Dockside and at
anchor? Or
offshore? Are you a sustenance cook or more culinary?
In my last house I had a great six burner cooktop. I miss that thing.
The biggest issue is balance in the gimbal. If you cook dockside/at
anchor, most of the time you can keep the cooker locked and use three or four burners. Underway in any kind of sea at all with the cooker gimballed you'll continuously be trying (and failing) to manage the balance so the gimbal keeps the cookware contents "level."
I bought a three burner cooker to replace the two burner that came with the
boat. After one trip I put the two burner back in and
sold the three burner.
Regardless of the number of burners, clearance between cookware is a problem. I've found a "fourth pot" like
https://i.ebayimg.com/images/g/rkAAA...Xb/s-l1600.jpg helps a lot. I can fit my 16 qt pressure canner and a fourth pot at the same time. A cast iron grill/griddle that spans multiple burners helps also for many
meals.