As Diego mentioned there are several factors that affect sizing.and you should work with a sales engineer to determine what is right for your environment.
With a v6500 having a max tps of 18000, you really only want to run it at 90% or 16200 tps.
Theoretical load is based on cpu count times 100, so on the gw you could have 162 cores.
Theoretical load is not accurate, I have seen a 2 core AIX server have more throughput than a 16 core Windows server.
If you have clustered databases as active/passive then that could reduce overall load as only one is running at a time.
In gateway clusters the total theoretical load is (# of gw's -1) * gw tps
And all of this is affected by database query and response size and query rate.