We just saw the Finding Nemo show last week, traveling with a wheelchair.
The wheelchair/
ECV seats are in back row of the main seating. There is an aisle and then there are some rows (I didn't count, but less than 10 rows) behind the back aisle.
I tried to count on our way out and it appeared that there was probably space for about 16-18
ECVs or wheelchairs. In addition, there was space on the side aisles where someone using an ECV would be able to transfer to the theater seat and then the CM could move the ECV somewhere else. We arrived in line about 30 minutes before the show started. We were in the regular line for more than half of the time, then everyone went thru a gate area and at that point we were directed to a wheelchair gate with our whole party. The person there asked if DD could transfer and then she radio'd ahead to a CM inside to say she had a 'non-transfer', so my guess is that they try to have people transfer out as much as possible to save spots for people like my DD who can't transfer.
The park map says to arrive 45 minutes before the scheduled show time for seating. I think that might be when they open the doors and begin seating.