I'm a bit confused. Some people say only one Darknet MN per IP address, others say there is no such limitation. What's the deal?
The deal is: It's not very well documented and most people simply do not know.
It also depends on how you want to run your Masternode's.
Most VPS cloud instances will provide a single static IPv4 address (some providers still offer shared NAT though). Increasingly, VPS hosts are also providing IPv6 addresses, which would allow you to 'bind' multiple daemon's running on a single host to each IP. As, far as system resources allow.
For IPv4 VPS hosts (or even your 'home' network), you could simply use a 'free' Tunnel Broker to add a secondary IPv6 address ;
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https://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokersThis method is known as IPv6 - in - IPv4
It is also possible to 'bind' to your unique addnode .onion address, when using Tor. However, binding to other Tor .Exit node operators IP's to run your own Masternode though Tor would certainly not be a good idea, as nodes will simply conflict, if everyone started doing that.
Basically, you want to own or at least directly control any 'IP' that you are going to 'bind' your Masternode to.
Hence, one DNET MN per IP address, is the best practical option.