What would be the maximum recommended spanning tree instances on a 6500 with SUP720's running native IOS? Is there a certain threshold or best practice as the maximum number of VLANS running on a 6500...before things get too chatty?
You have to configure each switch in the MST region to use the multiple spanning-tree protocol. Currently no method exists to distribute configuration to several switches, as far as I know.
I do not recommend to migrate your production network to MST on the fly, as there may be problems or network outages while you configure each switch.
Instead, I would make a sample configuration in a lab a would play around with it until I am familiar enough with its configuration and operation.
Then I would migrate the network out of working hours, with the possibility of rollback to the old configuration and possibility of thorough tests of the new configuration.
The configuration you have to do on each switch is the following:
spanning-tree mode mst
spanning-tree mst configuration
name xxxxxxxxxxx - this is the mst region name, this should be identical on each switch
revision X (X= 0 - 65535, this should be identical on each switch)
instance 1 vlan 1 - 30 ( this will assign vlan 1 to 30 to instance 1)
instance 2 vlan 31 - 60 ( this will assign vlan 31 to 60 to instance 2)
Remember, that this is just a sample configuration. You need to adapt this to your network.
The MST configuration commands are very similar to the traditional spanning-tree commands. For example to change the priority of a switch on intance 1 and 2:
spanning-tree mst 1 priority xxxx
spanning-tree mst 2 priority yyyy
With these commands you will be able to select the root switches for each instance.
This link will give you a good overview of MST basics: