Don't want to write this by hand? GodCraft sets minecraft:is_stackable for you with a switch.Get GodCraftOverview
minecraft:is_stackable is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Makes copies of this entity bump into each other so they can stack on top, like boats. Each one needs minecraft:collision_box.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:is_stackable": {}
}