It's quite straight forward. You can stack either potions or splash potions or both through config entries.

There is only one command - to reload the config (i.e change how many potions can be stacked together):
potionstackerreload - reload config ( potionstack.reload perms needed)



PS:
Should work for all versions between 1.8 and 1.13.2 as well as between 1.13.2 and 1.21, but I haven't explicitly tested this explicitly.