When I first started, yes, one per pen. But after a while I found converters to be a PITA so I switched to just (re)filling cartridges with a syringe.
Funnily enough, I STOPPED using cartridges for the same reason -- I found that flushing and refilling them was more of a PITA....
I try to have converters for every pen I have that needs them. A couple of times I've had to replace them -- the converter for my Jinhao 599 leaked, but I ended up using the replacement one for a pen I was giving away to someone. So now I'll have to get a replacement.
Most of my c/c pens take proprietary cartridges, so I have to be careful what converters I get.
Ruth Morrisson aka inkstainedruth
"It's very nice, but frankly, when I signed that list for a P-51, what I had in mind was a fountain pen."