I use this for up to 30 virtual MIDI ports.
https://www.nerds.de/en/loopbe30.html
It's perfectly stable on my machine. I use it to loopback MIDI data streams for an external MIDI utility called Vienna Symphonic Library Performance Tool.exe. Since the utility sits outside of REAPER, I need to connect these virtual MIDI cables to its inputs and outputs in order to send MIDI from REAPER, to the utility to get filtered, then back into REAPER, into Kontakt.
You can also use virtual MIDI ports to send MIDI data to other DAW software, or to standalone software instruments.