You can use as many as you want per turn, the main issue I see with this deck is Slugma feels like a constraining factor. Ideally you want 2 GX and 2 normal Maggies in play so you can get 2 uses of each ability, plus a manual attachment so you can stream attacks turn after turn. Turtornator will be a good way to get a lot of fire energy back in play, there's also the Firestarter Blaziken of course but that's a stage 2.
You need to drop all 3 energy to hit the 200+ mark, this basically means you need 2 uses of each ability and a manual attachment each turn to hit your key number, 150 isn't good enough against Buzzwole, Ultra Necrozma or Zoroark, but it does KO Rayquaza-GX with a choice band which is likely to be a strong deck post rotation.
Against 190hp cards, this combos well with Turt-GX's Bright Flame, even just having one of each down lets you use it turn after turn for one manual attachment, and you can probably be hitting that number from turn 2 on.
Shuckle is also a great card for Alolan Exeggutor, in fact I think that's the key card that Eggy/Garb needs to break into true standard viability since it lets you easily search out the x1 energies you run for shake, then with either ultra balls or sightseers you can dump a psychic and grass out of your hand to hit max damage much more consistently, creating the early board pressure that Garbodor needs to be useful in the lategame.