Merc, Mount and Familiar potions do not stack. They're marked with a number which means they are supposed to. Should be a quick and easy fix so please do. Thanks.
since this is low hanging fruit (imho) the devs may have decided to have them not stack for it to be a big trade-off in stockpiling these items
Any item that has serialized (temporary) data, such as these training potions cannot stack, as a stack is a single object with a count.