The answer is 13 blocks.
Because at first the scale is already balanced with a bag and 5 blocks on the left tray and 18 blocks on the right tray. And if you remove 5 block from both tray to keep the scale balance, the remaining blocks on the right tray will be 13 and the remaining blocks on the left will be zero block and a bag.
So if there are 13 blocks inside the bag, the balanced scale will still be balanced.
