How much refrigerant do these things hold?

[davepl]

I ordered this manifold set to vacuum down my AC system, and because I've never had an AC manifold hose set:

Amazon.com: Mastercool (89660-PRO5) R134a...
Anyway, in the event I discover that all my parts and o-ring replacement worked (ie: it holds good vacuum perfectly) should I at that point go ahead and add coolant, or if I've got that far should I just do it myself?

If that's the case, do I need the big propane BBQ tank size or is it practical and cost-effective to use the R-134 cans from the local auto parts store? I don't know how many spray-can size cans these would take, so maybe it's a silly question.