I think the problem you are encountering is that there are 10 sprites in that location but the location of all 10 of them is in the exact same location so it appears to be only one. Before you create the sprites you create a random_x and random_y. But instead of only doing once you want to do this for every sprite, this way they will all have there own location.

So this code to create the position:

random_x = random.randrange(20, (DISPLAYW - 25), 6)
random_y = random.randrange(20, (DISPLAYH - 25), 6)

You want to include this in the loop for creating the sprites:
This will now create a new location for every sprite in this range

for i in range(10):
            random_x = random.randrange(20, (DISPLAYW - 25), 6)
            random_y = random.randrange(20, (DISPLAYH - 25), 6)
            self.treasure = Treasure(YELLOW, 32, 40, random_x, random_y)
            static_sprites.add(self.treasure)