The ghosts are actually already limited to 8 at a time :P The Game Boy can have 40 objects (read: anything represented by a sprite) in memory, but it can only handle 10 sprites on any given line. The player and the bullet are both sprites so I capped ghosts at 8 so I'd never have to worry about it.
That said, having it feel good is a lot more important than hitting a specific target number, so that's definitely something I'll have to play with when I go back after the jam and tweak it.
What pains me is that I even considered capping your switch speed while I was developing the game, but the only playtester (me) didn't end up playing in such a way that it was necessary. *sigh* It's like I like to say: Show me a game dev who says "damn, we have too many playtesters" and I'll show you someone who forgot they promised food.