Hi, thanks for feedback!
Your analysis is right, in many games you usually recycle the enemy sprite to make an explosion, and reduce the total sprites count . But for Yo-Yo Shuriken, it wasn't possible, because when an enemy is killed, its sprite is reused to display a coin (or a bonus). The enemies always drop a coin or a bonus, because it's the only way to score points in the game.
So I had to use an extra set of hardware sprites to display the explosions :). It was more convenient because killing an enemy can now trigger more than one explosion at a time (like with the bosses).
So yes, for this game I do have up to 24 enemies (16x16 pixels) + 10 explosions sprites (32x32 pixels) on screen.