Based on the specs I last saw, it's a 70,000 square foot facility which can house 8 full size basketball courts or 16 volleyball courts, depending on configuration, plus a gym/performance center. I think they're also putting in sand volleyball courts outside.
The primary customer/user for a facility like this is amateur youth leagues in basketball and volleyball ("club" sports). Volleyball will be more focused on 11-18 year old girls, with some boys teams as well. Maybe an adult league but that would be minimal. Basketball would be a combination of youth and adult leagues. All the youth sports would be club team practices and tournaments.
People who aren't involved with girls volleyball have no idea how serious it is. This club (one of the largest in the area) has about 35 teams: http://www.ascvb.com/asc.html. Those teams all practice at least twice a week from November-May, and then will typically play in 6-12 tournaments in the Spring. So they need a LOT of court time when you add it all up, and they pay for that time. Parents of club volleyball and basketball kids pay anywhere from $2k-4k for a season, much of which goes to pay for facilities use. I'm guessing this facility could stay very busy on the volleyball usage alone, but having the combination with basketball will allow them a lot of flexibility. Adult leagues in the Summer for instance, when volleyball play is light. Etc.
To answer your question about spectator capacity, there is not a lot. Basically these tournaments are set up where maybe 50 people get a seat to watch any given game. Typically it's only parents and other teams in the facility during a tournament, so the object is to squeeze in as many courts as you can get, without making it downright uncomfortable for spectators. Some do a better job than others. Bigger tournaments might have more seating capacity for certain courts, but the purpose of a facility like this is definitely not to house spectators to watch a single game.