How many NAT mapping rules (that is, mappings between internal, private IP:PORT, and externally mapped IP:PORT) can a common NAT box support?
Is there a limit in such number (that is, how many rules can be 'active' at the same time) ?
How many NAT mapping rules (that is, mappings between internal, private IP:PORT, and externally mapped IP:PORT) can a common NAT box support? Is there a limit in such number (that is, how many rules can be 'active' at the same time) ? 


Two factors impact the maximum possible number of NAT mappings for a single device: the type of NAT and the amount of memory available for the NAT table:
I think that should give you a pretty good idea of the dimensions limiting NAT mappings in practice. Obviously there are various other factors along this dimension, like process size limits for userspace software NATs, or policies limiting the maximum number of mappings per single host, etc. 


NAT is just a mechanism, it doesn't define any limitations, and any such would be entirely implementation dependent. I doubt if anyone could even give a ballpark average as there are so many NAT implementations out there. This question doesn't really make sense. 


Up to now, the best answer is in this web page: http://www.informit.com/articles/article.aspx?p=25273&seqNum=5 :


