I have just learned the hard way that there may be some limit to your max number of spells?
I bought a new spell at a store, and next time I checked my spellbook my Flare spell was gone! How does that exactly work?
You probably just missed it while looking for it, it happens a lot of times with me when I have too much spells. Just look for it with attention, you'll find it eventually. I don't think you can have a max number of spells if you can have all the spells of the game just by typing PSB in the console.
Do you have any mods installed that might mess with your spells, if you're playing on PC? If you have only 8 spells, it shouldn't be all that easy to miss a spell... There's no maximum amount of spells you can have that I know of (and if there is it's definitely not 8), because I have characters with 100+ spells and they're all still there.
It may be under a different name?, some mods change the names and others you have to run the game through the OBSE_loader to use. Which spell was it, and whom did you buy it from Phoss?