1) 8 switches in a stack (0-7). If you need more than 8, you can create a "sub-stack" which a separate stack that is L2 connected to the main stack (you can use DAC cables at 10G, they will be switchports instead of stacking ports). Just don't add too many or you risk exceeding the MAC tablesl
2) You can add the switches, but they will remain dormant until you upgrade the code.
3) Yes, the primary (which is one of your existing switches) has control of the config. When you add new switches, you simply start configuring the new interfaces. The format will be <new-sw-#>/0/<port>
You can verify stacking with:
show stacking members