if you add a blank worksheet at the start and end of your data sheets, and call them Start and End, then when you enter your calcs, you could just enter =sum('start:End'!C12) or whatever your references are. this way, you can add or remove sheets as needed, and it will add across all the sheets you need it to, and keep the totals as the data sheets change. make sure your "Master" sheet is before the Start sheet though

Pull the sheets out of that range before you hide them