getpecked,
The short answer to your question is: yes, you are correct about the cartomizers. Although I recommend having extras on hand. The number you should have on hand really depend on you and how often/much you use them. I would probably recommend having at least 5 on hand and 10 would be better. They are considered disposable products, however that should not deter you from filling them multiple times. There is no set answer as to how often you should need to change them out as that is dependent on many factors (how much you use them, what juice you use in them, ect,). If vapor drops of drastically or its starts to taste foul even when its (mostly) full, then I would toss and use a new one.
Depending on your vaping habits, you could fill several of them up, charge a couple (maybe even one) battery and you would be good for a day (again, depending on how often/much you use them)
Hope all this was helpful and welcome to ECF.
Cheers,
VMS