I don't know the answer.
The first limitation would be the number of supported clients. You can see this in the MX sizing guide.
I would think you could safely allow for half a dozen NAT's times the number of supported clients.
They may be a physical GUI limit on how many you can type in and still be manageable.
Then there may be a hard limit in the GUI.
Well I've given it a try using the API and I was able to succesfully add about 2200 port forwarding rules to my MX64. Anything above that timed out or gave me a bad gateway error.