This Article is up to date with Custom Stuff 2 CS2

The maxstack attribute is a block and item item attribute that determines how much one stack of that block or item can hold. This attribute can be set anywhere from 1 to 64, including both values. Most items in minecraft are usually set to 64 so that they can easily stack, as do all blocks. If you do not want an item such as a new pickaxe to stack, then the maxstack attribute should be set to 1.

This attribute is added to your it's .js file. Some examples follow:

maxStack = 1;     //The item will not stack
maxStack = 16;    //The item will stack up to 16 items in total (like snowballs)
maxStack = 64;    //The default - Up to 64 items can be held in a single inventory slot.

Older Examples and Information for Custom Stuff 1.
maxstack = 1;      //the item will not stack with itself
maxstack = 16;     //a stack will only contain up to 16 of the item
maxstack = 64;     //a stack will contain up to 64 of the item