Most aim for 4, but will stretch to 5. Sometimes this is a limitation to what the boat can legally hold, but mostly I think it's just not optimal ... less fun because the timing is more spread out (taking turns on hits) and also more work for the guide, if the goal is to attempt to catch a limit vs a set number of hours on the water.
When Swiftsure is hot, it makes little difference, as the action is very steady. But on a slower day, let's say where there's only a hit every once a while, with 5 guys it can be a long time until you touch a rod again. Been there, done that ... play your fish, then go have a nap for a while. ;-)
PM me if you want me to contact some renfrew guides on my list on your behalf.