On the production side, you figure out what you are projecting to produce, you figure out the average time the beer will spend in the fermenter, you figure out what your batch size will be, and you solve for x.
You project that you're most likely to produce and sell 1000 BBL a year. You're brewing 7 BBL batches. Your beer spends, on average, 14 days in the fermenter. That means, at its maximum, every fermenter can ferment 26 batches.
26 cycles * 7 BBL = 182 BBL per fermenter, so you would need at least 6 fermenters to produce 1000 BBL a year in that setup.
The other tanks depend on what your overall business plan is. Is it a brew pub, is it a tap room, is it a distributing brewery, etc. And the fermenters can vary based on the same things. Maybe you project that you are going to make and sell 1000 BBL a year but that 750 BBL of that will be of the same beer. Then you would want to double-batch beers, maybe you would get 2 15 BBL fermenters and 2 7 BBL fermenters.