It should be about capability, not crew size.
May I offer a perspective? It seems that many aircraft manufacturers have expended a lot of effort to eliminate crew members, like navigators, radio operators, flight engineers, etc. The F-35 was even specifically designed to be solely a single crew airframe. There is no version with two seats. So crew size does seem to be an important criteria for many military aircraft.
The existing P-8 architecture is pretty flexible. So adding sensors does not necessarily require adding a console and operator for those sensors. An existing console could be used. So there could be different guys trained to operate different sensors from the existing consoles. If the mission is primarily anti submarine, acoustic operators would be aboard. If the mission is primarily electronic surveillance, a different operator would be aboard. Basically, you'd build a crew that had the expertise/training for the specific mission. The downside is that while the aircraft remains multi-mission, any individual aircaft would be crewed to only do some of the missions. I don't know if that would be good or bad.