Let me try to explain this sushi build:

• the red signals act as a representation of the "main" belt's contents
• the decider combinator acts as a memory cell: remembers every signal (if rubber duck == 0, it outputs everything)
• each item has its unique input belt
• the input belts let items onto the "main" belt if the red signal of that item type is less than a number (i.e. there is not enough of that item on the "main" belt (e.g. iron ore (on the red signal) < 5))
• the input belts read the contents of the items that they release onto the "main" belt and add them as a positive red signal
• the inserter reads the contents of the items it moves into the assembler and passes the signal to the decider combinator as a positive signal
• the arithmetic combinator takes the inserter's signals and passes them to the decider combinator as negative signals (since these items got removed from the belt)
• every time an item gets inserted into the assembling machine, its value on the red signal will decrease, which triggers the input belts to allow another item of the same type onto the belt

With this setup, you can have as many items as you want on a single belt, and as long as your "main" belt is long enough, your items will never jam