So how many extra CO2 cartridges do you carry with you on a trip?

I wear these inflatables for other on the water pursuits but not canoe trips. I have a waist belt style (Mustang)and a horse collar style (Stearns). I've always wondered how many cartridges I would bring if I did use them on a trip and then realize the other uses that a standard PFD offer. The cartridges for the waist belt style cost nearly $40. The auto deployed Stearns horse collar style also requires activation tablets that MUST be kept dry. I thought that would be a PIA.