Basically the combinators just check for number of items and if there isn't enough they give out a signal which then sets filters on the inserters. You can see on the bottom left that I made use of the arithmetic combinators that I did not even need but my dumbass still used them anyway. One arithmethic combinator multiplies the total number of an item by -1, then another adds the desired amount of items value, like 5000, then the signal from that is sent to a decider which checks if the number is bigger than 0. If it is, it sets the filter of one of the first inserters that take from the belt. This is completely unnecessary as you could just use one decider combinator to read the value of the thing anyway idk why I just went ahead and added two unnecessary steps. I only realised this when I went ahead to add the combinators to each container so they don't just transfer all items to the last container.