What kind of applications are running over this network? Is this mostly telnet, or are you doing lots of multimedia/video/voice? MSFCs or external routers?
The short answer is that you can put 7x16 GE ports into the 6509 switch, so you could feed 112 switches. This would work fine for telnet-type traffic, but multimedia might suffer. (NOTE: This is reserving 2 slots for supervisor modules.
A better answer is 5x16, allowing 2 slots for supervisors and 2 slots for switch fabric modules, which will increase your performance. Additionally, you can use Etherchannel to bundle multiple GE interfaces to each 400X switch.
HTH...