1) Depending on what the overall finish is going to be (brighter or more subdued), I use either white or medium grey spray primers for the figures.
2) I usually always prime 20-30 figures at once. I don't keep exact scores, but I would estimate I'd get at least 8-10 sets out of a can, so between 150 and 300 figures. Mind that I only do a single spray primer coat and then, after it's dried thoroughly, cover that with a brush primer coat in a tone most suitable for the desired finish.