That's what dynamic DNS services are for.

Also there is no practical limit to how many masternodes you can run on a single public IP address especially since all the masternodes (at least that I had the opportunity to run) do not need to be exposed to the internet at all (ie can be behind NAT, no need for any port forwards).

Again, my example might not be the right one for everyone since I have several servers and do stuff that are not always crypto related but I definitely prefer this type of setup simply because I'm in control over basically everything.