No hard limit on MNs but my calcs indicate it will level out at 2,000 MNs:
For some reason I really thought Evan said there'd be a hard limit to masternodes, either 400 or 4000, but I can't find the post.
He said it, I'm sure. But I think it was a very high number that he did not expect to reach because there would not be easily available coins. The number that comes to my mind is 10000 but I'm not sure at all.
If you want a hard limit, there are only 4.3 Million darkcoins out there, implying max MNs of 4300.