I expect a mod will fix this later on.
If I remember correctly didn't Oblivion only have 8 hot-keys too? Or was it 9?
I kind of like that even if there isn't enough hot-keys (they could've included 9 and 0 as well) you do get the benefit of the favorite menu where you can consolidate a lot more spells, potions, well anything you use a lot in battle.
Quicker than going to each sub menu all the time.
Oblivion had 8, Morrowind had 10 but you needed to hotkey lockpicks and probes to for dungeons.
In Oblivion I managed with 8 as an fighter, bow, sword, healing potion, poison, healing, summon, still left room for soultrap and extra weapon or frenzy.
As an pure mage I had to use much more because of the weakness spells and I wanted to do fire and shock against different enemies.