It could be Standing Room Only. Do try to get a FP and you'll be seated in the reserved FP section - the left side of the amphitheater. If you are unable to get a FP, plan on arriving at least 90 minutes before the 1st Fantasmic.
Two years ago on New Year's Day, the first two Fants were Standing Room Only. After the first Fant, we came out of the amphitheater and the line for the 2nd Fant was already to the Hollywood and Sunset intersection.
If there are three Fants, you may want to go to the third show.