There are tons of single bolts laying around from where my dwarves have shot things. If I reclaim them will dwarves restack them? I don't really want my dwarves running around with 1 or 2 arrows on them.
As of the current version of Dwarf Fortress, Dwarves and Adventurers do not stack things. Ever.
If the bolts are metal, it might be better to melt them down instead so that you will eventually recoup some of the metal for a new stack, because the alternative, as you yourself mention, is a dwarf running around with a 1 stack of ammunition.
In the latest version of Dwarf Fortress, bolts don't stack. But your dwarves will store multiple bolt items in the same quiver so they will have an adequate supply of ammunition.
The lack of stacking does mean that when you want to move bolts from A to B, it will take more hauling jobs to move the individual bolts.