Well how deep do you want it to be? If (for example) you want a 4" depth over the whole 8x20 run, that is 160 sq ft times 4" = 160 sq ft times 0.33 ft = a little over 50 cubic feet. (Less than 2 cubic yards, if you want to use that unit instead, as most aggregat companies speak in terms of cubic yards not cubic feet)
Googling seems to suggest (i.e. you should check this for sure yourself
) that sand weighs about 100 lbs per cubic foot. That would be 5,000 lbs of sand. Which is, what, like 2 tons and a bit.
Mind you that's for just a 4" depth. You want 8", it'd be twice that; you want a foot depth, it'd be three times that; and you REALLY REALLY should check my math and numbers because I assure you I did not have 5,000 fingers to work that out on
Note that if you want it deeper than 4" or so, you are really going to have to think about ENGINEERING whatever boards are going to retain it around the edges of the run, because even just 6-12" depth of sand exerts some serious horizontal force. It is not that hard to do, but would *need* to be done if you don't want things busting and/or sand slouching out all over as time passes.
Pat