what is max number of Catalyst 9300 switch can be stack together?

[Herman2018]

hi, we have 4 new C9300 switches which are already stacked together and work as core switch. Now we have other 2x old C9300, and want to add them to the network. what is the best practice of below two options? 

option 1. upgrade the os of old C9300 to the same version of new C9300, then add them to the existing stack

option 2. just stack 2x old switches and uplink connected them to existing 4x new switch stack using port-channel, this 2x old switches just used for non-important endpoints. 

what is pros and cros of stacking 6 switches together? Can anyone please advise, thanks in advance!

[shambhu.kumar]

Use option 1

- Verify the compatibility and make sure the model is supported to add a stack
- Update firmware with the same software version as the existing stack members. All switches must either be in Install Mode or  Bundle Mode
- Backup configuration
- Connect the new 9300 switch to the stack using the appropriate stacking cables.

- Power on the new switch and verify that it boots up successfully.

- make a note of exiting stack switch numbers and priority and Use the appropriate command on the stack master switch to add the new switch to the stack.

- Once the new switch is added, verify that it has successfully joined the stack. Use the "show switch" command

https://community.cisco.com/t5/switching/c9300-add-new-switch-to-stack/td-p/4386257

[Herman2018]

thanks @shambhu.kumar for your advice. can you pls advise the pros and cons of option 1? why don't use option 2? any disadvantage if too many stack members? thanks.