I think the question you are asking is "how many seats will a single serving tank supply?".
DISCLAIMER: NOT A CURRENT PRO BREWER NOR BUSINESS OWNER
I think what you need to look at is what the % of sales that particular serving tank will be. You would need to have an idea how many beers/seat and what the turnover rate is. But below is some basic math I did (note the disclaimer)
75 seats * 32 ounces/hour * 1.1 spillage factor * 1.025 sampling factor (guessing) = 2706 oz/hr = 170 pints (16oz)/hr * (1 gal/128 oz) = 1.31 gal/hr
if you serve between 11am and say 11pm closing, thats
1.31 gal/hr * 12hr = 16 gal/day * 7 days = 110 gal/wk = 3.58bbl/wk
This is a ballpark estimate. Try to find the spreadsheets from Republic Brewing (search for it on here). There are some very helpful calculations in there on sizing serving tanks and how much to make depending on how many seats you have. 6 serving tanks would be okay if you could sell the beer fast enough. Your sales would be split between the 6 and that means it takes longer to empty said vessels. This also plays into your brewing schedule and your business plan.
Homebrewer/Future part-time brewer
but I do have 1 professional brew under my belt and on the books, and its still on the menu at that particular bar even though its not being served right now.