Welcome to the forum! It sounds like you want to build up enough lacto to sour a 12 gallon batch. I do think you need a starter for this much wort and you will be more successful with pitching enough bacteria. The last BW I made (for 5 gallons), I started off with one WY lacto pack in a small 1 qt starter of a low gravity (1.020), unhopped, no aeration (i.e. no stir plate, use an airlock and don't swirl like a normal starter). Keep it warm (around 90-100F) and let it completely ferment out (about 7 days). Since lacto is very dusty and doesn't like to settle out well, I chilled the lacto overnight to help it floc to the bottom of the flask. The next day, let it warm up to room temps, carefully decant the spent wort on top, and add 2 qts of 1.030 wort to the flask minimizing aeration the best you can (obviously a little splashing will occur and thats okay). Slap the airlock back on and keep the temps between 90-100F until the wort is completely fermented out (7-10 days). After fermentation, cold crash overnight to drop out the lacto, then bring up to room temps on brew day, decant and pitch.
That was enough lacto to successfully ferment out a sour berliner for 5 gallons. If you need to double it, then by all means do so. I would double it after the 2 qt starter completely ferments out. That is when I would step it up to a gallon.
Keep in mind there are many ways to skin this cat and I can only tell you what I have done and had success with. That Berliner ended up taking a 2nd in NHC regionals a few years back. YMMV. Good luck and be patient as you will be rewarded!
Cheers-
Brewinhard
