The SRND is the main source of advice and formulas for calculating CTI impact: https://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/uc7_0.html
 
Also available for CTDP partners is the UCM Capacity Tool: http://www.cisco.com/cgi-bin/CT/PGWCT/ct.cgi
 
(Request access via developer-support@cisco.com if you are a current CTDP partner.)
 
Some rules-of-thumb:
 
MCS 7845 hardware
---------------------
2500 - CTI devices per CTI connection (one JTAPI or TSP connection)
2500 - CTI devices per UCM node
10K - CTI devices per cluster
 
Non-7845 hardware
---------------------
2500 - CTI devices per CTI connection (one JTAPI or TSP connection)
800 - CTI devices per UCM node
3200 - CTI devices per cluster
 
- Each CTI connection counts as approx. one 'device weight'.  A PC with Outlook+TSP controlling a single line phone counts for '2'.  A JTAPI app controlling 100 devices, counts for '101'
- CTI counts are per line on the device.  Monitoring a device with 4 lines counts '4'
- Registered vs unregistered does not matter, however 'base' usage assumes 6 calls per hour.  Devices which see a lot more action/complexity (CTI route point, call center agent's phone) will have a multiplied weight.
 

- One CTI connection (a Provider in JTAPI, a TSP instance in TAPI) can manage up to 2500 devices
- One CTI connection - by itself, no lines/devices - is equivalent to one CTI line.  So, one MCS 7845 (normally supports 2500 CTI devices) could support 1250 separate CTI connections, each monitoring one device
- If an application needs to monitor 5000 devices, then it will need 2 JTAPI Providers or 2 TSP instances
- Note a phone with 5 lines will count as 5 CTI devices
- A CTI device counts each time it is monitored.  If 500 devices are monitored by two separate CTI providers (500 devices monitored twice,) the weight is 1000 devices (plus 2, one for each provider's connection itself)