AW IMR for all of your mods, easy breezy folks. Safety first. I bought mine from provape. 18350, 18490, and 18650 all work with the SVD and you don't even need the spare tube for the 18650 configuration. Plus the SVD allows you to stack 2 18350's if you wanted to, just make sure you reset the configuration if you do this (it's all in the manual).
As far as how many, answer is more than 2 or 3.
RHP