For split screen, I use Davinci Resolve like Zoot mentions, but I paid for the full version to enable all the effects and capabilities. Be aware, lots of processing power and RAM will be needed to handle 8+ 4k resolution videos on the same screen. Thats about where my computer gives out, and it's got some good high end specifications.
OBS Studio is what I use to stream live for YT. Previously, I used Shotcut, which is great for as it is completely free with all features available. I did find it stuttered and skipped a lot with more than 6 videos though, which is why I upgraded to Davinci.