There may not be a hard number on that, but always consider quality over quantity. And these guide questions:
IS IT USEFUL?
The way skills are designed in MV and Ace, output can be set to be proportional to character stats keeping them relevant even as you progress through the game. There isn't a need for a character to have seven different kinds of single target sword skills. It's redundant. Unless said skills have unique functions to them outside of causing damage, they're just there for the sake of being there. Perhaps one has an elemental affinity? Or, it stuns on hit? Think of skills as specialized tools designed to solve problems you as a developer have presented to the player, and not as giant clubs players just use to smash at things.
IS IT BALANCED?
Usually in RPGs with more than one playable character, each unit serves a particular role. Perhaps one unit specializes in dealing a lot of damage but has horrible defense, or another unit is a little more durable but mostly only functions to support the rest of the team? This isn't a hard rule in RPGs, especially when players are given the freedom to build characters however they want, but even then going in one direction ought to come at a price: that skill point you put in strength could have gone to defense, for example. Having a unit excel in all aspects across the board runs the risk of invalidating the need for a party if we're talking about just one unit and making the game far too simple if we're talking about all of them. It's possible in some games, and only as a reward toward the end for people who've done some serious grinding.