2-3 tubes should be plenty...but I highly recommend getting more. Once you start using the stuff, you find it's really wonderful, really indispensable, for a jillion things. I used it to put new hinges on my daughter's toy oven a few weeks ago, nothing holds as well as this stuff. It's just great to have around.

I keep thinking I waste MORE mixing it in cups and on scraps than I do using nozzles. There is always some left over on the scrap or cup after you have finished mixing. I have tubes several years old, kept unrefrigerated, that seem as good as new.
I get mine, and the six inch nozzles, from Art's Hobbies. I got the gun from BVM way back when. The gun and nozzles, to me, are actually a very ECONOMICAL way of using the stuff, you get the right amount of glue right where you need it, and there is not THAT much left over in the nozzle when you toss it out. Not enough to really matter.
Like someone else posted above, the best way to use it is to line up a bunch of jobs that need hysoling, and leave them all for the last thing you do before you finish up at night. That way you only use one nozzle, and it's all dry when you get back into the shop the next day.