The GPU won’t have any issues encoding several video streams at the same time. That’s not really necessary though. The cameras will do the encoding for you. Just set the bitrate and framerate that you want on the camera and pass that through. Most cameras support two streams, the secondary stream is usually limited to SD though. All of my Hikvision cameras support RTSP. It’s mostly just the consumer grade crap that only works with the manufacturers cloud service so they can spy on you or restrict features whenever they want.

Don’t make the mistake I did and buy any Hikivision stuff from Amazon. They are all grey market and the firmware can’t be updated. I tried to update one of mine and now the user interface is stuck in Chinese. You have to get them from an authorized distributor.