Is there a maximum amount of energy that can be attached to one Pokémon?
There is no limit.

Each turn you can only attach one energy to one of your Pokémon (either a benched or active Pokémon), and there is no rule limiting the total energy.  

The downside of lots of energy on one Pokémon is that if your Mewtwo gets knocked out, all attached energy is put into the discard pile.