A 6509 has 9 slots.
Things to consider:
What Supervisor engine to use.
The Sup2 will use one slot. If you want reduduant Supervisor engines, you will need two slots. The Sup2 only has a 32Gbps bus so if you expect more than 32Gbps flowing throught the switch (a distinct possibility if you have 110 Gigabit users) you may want to get a Switch Fabric Module (SFM) which will also occupy a slot and will provide 256Gbps of backplane. If you get a Sup720 it has a Switch Fabric Module built in and will provide 720Gbps of backplane capacity. (Note these are "full duplex" numbers)
After you decide what you are going to use for a Supervisor Engine you must now look at your Gigabit connectivity requirements:
There are 16 port fiber Gigabit modules and 48 port 10/100/1000 copper modules (There is also a 16 port copper 1000 module)...here's a URL listing 6500 ethernet modules
http://www.cisco.com/en/US/products/hw/switches/ps708/products_module_installation_guide_chapter09186a008007efee.html#1015439
Here's some info on the Supervisor Engines:
Sup720 FAQ:
http://www.cisco.com/en/US/products/hw/switches/ps708/products_qanda_item09186a0080159963.shtml
links to the data sheets on the Sup2 and 720:
http://www.cisco.com/en/US/products/hw/modules/ps2797/ps5138/index.html
There are many options for the gigabit ethernet connectivity requirements, copper, fiber, what type of connection they have to the 6509 backplane, etc.
Decide on the level of redundancy you need, what kind of supervisor engine you will use, then what kind of gigabit connectivity you will need and figure out how many slots will be populated and how many ports will be provided on each blade...
You could also think about a 6513 (13 slots). The cost isn't that much more for the 6513 chassis vs. the 6509 chassis.
-HTH
