Or even faster, visually split them into 5 stacks of 100. Assumes constant thickness, but then make them even by moving pages from the highest ones to the lowest ones until they are all identical. If you can’t make them all identical, then you don’t have 500.

Then split one of the 100s into 5 again the same way.

Then pick up one of the stacks of 20 and count it to confirm it is 20.

Or for an arbitrary number of sheets, do the same but then set aside any sheets that won’t even up (because the stack isn’t an even multiple of whatever number you divide it into). Then you just have to count the final stack, multiply that to compensate for the splitting (for that layer), add the set aside sheets (for that layer), then move on to the previous layer of splitting and repeat.

Or, count out 20 sheets, then measure their thickness, then measure the thickness of the entire stack, divide by the 20 thickness, multiply by 20, though this one will have some error due to the measurements not being exact (even if you use calipers, paper does compress a bit, which complicates exact measurements).