Depends on how important this feature is to the game and how big your maps are. If this is like Harvest Moon where NPCs are essential to the game, you should go all out and not complain about hard work (that's what makes good games after all, especially for RPG Maker games where standing out of the norm is essential for success). If it's like any traditional RPG where the NPCs just spout nonsense or tips, then you don't really need a whole lot.
You should follow a logarithmic curve where the x-axis is your map size and the y-axis is the number of NPCs you should have.