CUCM doesn't enforce any limit but applications are expected to be within the limited mentioned in scalability improvements slides at http://developer.cisco.com/web/jtapi/docs.
Look for
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Unified CM 7.1(2) CTI Scalability Improvements.ppt and
Unified CM 8.6.x CTI Improvements.pptx in the above link.
http://tools.cisco.com/cucst (CUCM SNRD) should help as well.
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