If you want a real accurate number, you'll need to measure it:

measure the depth of water in a line down the center of the pool every 1 foot (ignoring the stairs for now) , you should end up with 37 measurements (0 ft, 1ft, 2ft, .... 35ft, 36ft).

Calculate the "average" depth for each 1 foot long x 18 feet wide section by averaging the depth reading at start and end of that 1 foot section but subtracting maybe 2 or 3 inches for rounding at the sides of the pool. Then multiple this average depth for this section by 18' x 1' to get xxx cubic feet for this 1 ft long section

Do this for all 36 sections, add up all 36 values of cubic feet, multiply by 7.48 gallons/cu ft to get gallons.

You'll need to 'fudge it a bit for rounding in the deep end especially, and also the stairs, but this shoudl get you within 5% of real value