I'm not sure.
I did empty a workbench one day to test the limit but I forgot all about it.
I was reminded a couple of days ago when i returned to that settlement and looked in the bench.
It had been quite a few days and there were a couple of hundred items accumulated.
It's probably random what they "virtually collect".
Any item from A to Z.
teddy bears, units of wood, concrete, cork, leather, cotton, bottles , silver lockets and so on.
Perhaps you could do a test of your own.
From my point of view I have settlements with quite large populations so the settlers have to do something to keep them occupied.
The main reason i have higher populations is because I like to have the settlers do most of the settlement defence.
If you are building up settlements en-mass the scrapper benches are never going to be able to supply enough resources.
They are, however, useful for maintaining improvements.
I recently went around the settlements and reduced turret numbers, replacing them with guard posts and assigned settlers.
The scrapping of turrets gave me some resources to play with.
I also built lots of nicer beds and added lots of rugs.
Consequently I ran out of cotton, so made it a point to pick up as many shipments of cotton and items containing cotton during my travels.
I put the building on hold and went foraging again.