This is not an official mIRC website, and this page's wording is.. poor, the number N you pass in /hmake is a number of slot, it's not the number of item you can have nor a limit on the number of item you can have.
You have a good analogy of how hash table *works* here
. Basically, slot are just rooms, each room contains a list, and when you add an item, it automatically finds a room, then the item is added at the end of the list in that room. These lists are not limited themselves, only RAM and Windows will put a limit on how many items you can have, so these limits will vary from machine to machine, and depending on the items themselves