Hi David,
you can check the number of devices the router is configured to support with
show call-manager-fallback
but you'll have to check what SRST license was purchased with the ISR to confirm exactly how many devices you're licensed for.
Have a look at the following data sheet to confirm what your ISR supports and if it can support more than you're currently licensed for you can purchase multiple parts to reach the desired phone count.
http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/vcallcon/ps2169/data_sheet_c78-485221.html
regards,
Paul