Just tried it myself. At first it didn't seem like it does anything, because if you place one down and start adding signals, nothing gets multiplied by the stack size.
Here's how to use it properly: after adding one of the 0-9 blueprint parameters to the stack size combinator, when you apply the blueprint it'll multiply the item you selected with its stack size.
e.g if I have a blueprint of a stack size combinator and set it to output Parameter 0 with a count of 1, if I use that blueprint and select yellow belts as Parameter 0, the stack size combinator will output 100 yellow belts.
