We are planning on brewing 1bbl batches, but we are kind of stuck on how much yeast we need. We contacted White Labs but their 1bbl pitchable yeast is way too expensive. I want to know what would be the best way to reuse yeast from previous batches and how much do I need for 1bbl?
The best place to look for this info is www.mrmalty.com . The answer depends on the amount of slurry you have and its age, whether it's an ale or a lager, and the OG of the beer. The calculator on the mrmalty website will help you figure out how much to use.
In Yeast: The Practical Guide to Beer Fermentation, there is a propagation schedule involving stepping up each successive starter wort by a factor of 10, with a waiting time of 24-48 hours between each step. So you start with 1 vial and make a 1L wort. Let it go for a few days (depending on how you aerate it... a stir-plate would be ideal). When the first starter is done, brew a 10L wort and pitch the yeast from the first starter into that. you can increase again from there, but you're going to need successively larger kettles and fermentation vessels for each step.
I've done the single step-up (1L -> 10L) once before for a group barrel brew and that got me ~1/2 gallon of yeast cake.