The words "seat" and "seating area" are being used in 2 different ways through this thread. I know Fantasmic! at
Disneyland like i know my house and I am getting confused. So.... in an effort to help....
There are 2 types of "seating areas" for Fantasmic! at Disneyland.
1 - The seated area: Using the photo above, you can see the area to the right of the water where everyone is sitting. It goes along a good portion of the riverfront, from the river back to the rope you can see, and is my personal favorite place to watch Fantasmic. This is an open seating, first come, first get, sitting only, free, viewing area. They require everyone sits once the show starts. Not kneel, not squat, sit. In the very back row of this area, wheelchairs, ECVs, and strollers are allowed. The view is fine because everyone in front of you is - yep, seated. There is not an actual number of people that dictates the CMs cutting off this area, but if they see that it is full, they will not allow anyone own there unless your party is down there holding seats (as there's no point because there is no more space).
2 - Priority Seating: Using the photo above, this area is to the right where there are chairs. It is reservations required (through Guest Relations/City Hall), additional fee ($69 for adults, age 10+ and $59 for kids 3-11), assigned seats based upon checking in evening of, comes with a large dessert box (assorted sweets, cheese, crackers, and fruit), beverages (soda, water, juices, coffee, hot chocolate, etc...), and servers to wait on you. There is an absolute number of seats. Once sold out for the show, it's sold out. Seating is done for both shows. If you purchase Priority Seating for the first show, you are able to sit in your seat and watch the fireworks afterward.
TigerLaw, I believe you are asking about #1.
ballarinamom, you are discussing #2.
DangerMouse and HydroGuy are discussing #1.
APX is discussing #2.
Tigerlaw: I agree with HydroGuy. Finding a seat for 1 in the seated area should not be too tough. A smidge of kindness to others already in the seating area goes a LONG way when looking for a seat for 1. (I have watch Fantasmic! solo many times!

)
laura&fam: Yes, the fireworks main viewing area is Main Street, in front of the castle. However, the first Fantasmic! show ends 2 minutes before the fireworks show starts. Many people who watch the first Fantasmic! show will choose to watch the fireworks from the Riverfront where the fireworks can be seen.
Hope this helps,
Dreams