Depends. Are you planning on incorporating scripting and various plug-ins into your game, or do you intend on stock RPG Maker features?
I agree with the others in that you should avoid having "The Same but Better" spells, that do nothing that an earlier spell didn't do but with more damage. Sometimes, it helps to look to existing games for examples of what seems to work and what doesn't. For reference, I'm looking up "Final Fantasy Tactics" right now. It has the most abilities that I can think of in a single game, and from what I remembered, things worked out pretty well. FFT has the following number of skill-type abilities per job. (not including counters or passive abilities, just the equivalent of RPG spells/specials)
Squire: 4 abilities, no duplicates
Knight: 8 abilities, all various "breaks"
Archer: 8 abilities, all various "charges"
Monk: 8 abilities, no duplicates
Thief: 8 abilities, all various "steal -----"
Geomancer: 12 abilities, no duplicates
At this point, you get the idea. I really don't feel like looking up the rest and copying them here. Feel free to do that yourself if you want. Anyways, after a cursory glance, not accounting for overlapping abilities in multiple job classes, even when we only look at the initial six Jobs, there are a total of 48 abilities. If you were to compact things like "Break Helmet/Break Armor/Break Weapon" into a single ability with a secondary menu to ask what you're aiming at or how long you intend to power-up (Archer's "Charge" skills), you'd reduce the number of unique abilities down to 26. That's a pretty massive number of unique skills, especially taking the fact that we didn't look at the entire game.
FFT ended up working pretty well, and it had so many abilities. I think that we can safely say from this that players are easily willing to accept huge amounts of skills as long as they aren't "The Same but Better".
Now, back to my original question, do you intend on utilizing scripts or plug-ins? This could help you if you're still not certain. I've played Disgaea, and I remember having to sift through a countless number of "The Same but Better" spells, having a huge list to go through until I found the spells that I wanted. What you could do is have a "shortcut" list, or some sort of way to "hide" the less-used spells so the player has a more compact list of their personal choices at their immediate disposal. You could set it so hitting left/right would swap to a secondary page with every spell, and the initial page would be a list of favorites.
Another thing you could do would be to design your spell list, assign them to various characters, and make a sort of testing game. One of those ones with a number of generic dungeons surrounding a single hub town. You could make a full test game with minimal work on your end, and populate it with the enemies that your full game would have. Get a couple friends or other volunteers to play through the game, and ask them what spells they used most and what spells they never found a reason to use. Cull your lists appropriately, then take your revised version and use it as a base to build the game you want to build.
Off-topic: I've noticed that most posts I make on this site end up being solid walls of text. I should probably try to fix that habit.
