How do I make stacks of unstackable items?

Although the /give command enforces stacking restrictions, you can work around them by manually summoning item entities using the /summon command. For example, the following command will summon a stack of 64 diamond pickaxes, which will remain stacked to 64 when you pick them up.

/summon minecraft:item ~ ~ ~ {Item:{id:"minecraft:diamond_pickaxe",Count:64b}}

The Count value can be anything from 1 to 64 (stacks bigger than 64 are automatically split up into stacks of 64 when you pick up the item).

Note that this has some surprising results. For example, if you mine with a stack of tools, and use one tool up fully, then you are left with a stack of undamaged tools one fewer in quantity, but if you split the stack into two stacks, both retain their damage value.