Hmmm... Thanks, I don't have much rhyme or reason behind the numbers, but right now I was aiming for:
Bleachers: 8,000
Lower Tier: 2000
Courtside Seats: 500
Luxury Boxes: 20
After I hit those numbers, I'll probably split my focus between continuing to expand and spending more money on players.
It's good to know that DIV can probably handle more luxury boxes though... Maybe I'll up the # of those and reduce some of the courtside and bleachers.
edit:
Actually on second thought... I'll probably begin to split my focus before I those levels, but I'd like to get close to those #'s as soon as possible before making major player expenditures.
Having a plan already means you're probably ahead of the curve. ;)
The thing I'd also counsel is that bleachers are relatively cheap, and that the financial pressures become greater the higher up you go, so if it ever comes down to a choice between more bleachers and more anything else, with other things being equal, prioritize the more expensive seats. Obviously, the all other being equal part encompasses things like not building seats that won't sell for another promotion or two (but I need to point it out explicitly just in case).

As far as balancing players and seats, the longer you think a player could be useful for you, the more weight you'd want to give to him in your decisions. If you found a well-built ~25 year old player with a 20-25k salary that could star for you now, be a solid starter a level up and still play quality minutes no matter how high you go, that's the kind of player you'd love to have to show for your player purchases.