Hi, Quinn!
Fantasmic! in Hollywood Studios is set-up in a very large ampitheatre that seats over 6,000 guests. The seating is first come, first seated, so the earlier you get in line, the better your choice of seat. However, the way the show is performed, there really are no bad seats (just getting a seat is the trick!)
In addition, there is "standing room only" for an additional 3,000 guests. I have done the standing room option. Watching the show this way isn't the best: it is crowded and views are obstructed.
Even with a capacity of near 10,000 in this ampitheatre, seats are often all filled before everyone in line gets in for the show. I recommend getting in line for this show at least 1 hour prior to showtime to get a seat.
I hope you have a great June vacation and enjoy Fantasmic!