It's limited by the amount of DRAM in the unit.
http://www.cisco.com/c/en/us/support/docs/ip/network-address-translation-nat/26704-nat-faq-00.html
Q. How many concurrent NAT sessions are supported in Cisco IOS NAT?
A. The NAT session limit is bounded by the amount of available DRAM in the router. Each NAT translation consumes about 312 bytes in DRAM. As a result, 10,000 translations (more than would generally be handled on a single router) consume about 3 MB. Therefore, typical routing hardware has more than enough memory to support thousands of NAT translations.
