Steam locomotive boiler water capacity

I was curious as to how much water a steam locomotives boiler can hold say like a 4-8-8-4. Is it proportional to how much the tender carries?

Well, I do not know the volume of the boiler of those big US steamers, but for a series 01 or series 44 type boiler you can assume 10.000 litres of water inside in operation level, the tender was at 34.000 litres capacity.
There was no realtionship betwen tender water capacity and boiler water volume... so to run longer distances, simply take a larger tender... if you have enough time and good water points along the road, you can run with a smaller tender without any problems.

USRA heavy mikado: ROUGHLY 6,000 gallons (if my calculations are correct) after subtracting flues, tubes, dry pipe, boiler stays, firebox, etc.