You say that you don't know how many of each you sell because you are too busy. Here is a very simple way to your counting problem if you do not have a cash register with separate keys for each food item you sell.
Before each shift, count the buns for each sandwich (full size and slider). If you have any waste (less than perfect bun or, God forbid, a returned product), place them in a container to use for bread pudding or something. This is inventory that needs to be counted and not wasted. If you have returned product, put that in a different container so you know the count.
At the end of each shift, count the number of each bun left and subtract any waste you saved. You have your sales on each type of sandwich. Throw away any soiled bun waste and save the good bread for puddin you can sell for additional profit.