A similar question came up on the Unity forums a while ago, so I'll copy my answer here in case it's useful to anyone else.
One of the MMOs made with Mirror had quite a lot of CCU when it launched, I think it was Inferna. They split the map into separate server instances, with around 200 CCU limit per map. I would have to look it up again, but I believe they achieved around 1000 CCU per world this way at times. There's also a very old video of us trying 480 CCU worst case, all in one place like your video. It's laggy as hell, but the server survived just fine. Both Inferna and the 480 CCU video use old Mirror & Unity versions. We've had years of improvements since then, both for Mirror, Unity and server hardware. We still have plenty of optimizations to squeeze out of Mirror, with another CCU test planned afterwards around the end of this year. Keep in mind that your game's complexity is also a huge factor. A 3D physics movement MMO like WoW will be way harder to scale than a 2D click-to-move MMO. For an Indie developer, 2D is really worth considering. It's way cheaper, way easier to make and scales way better due to less complex physics/meshes etc. At the end of the day, there's definitely a limit to what we can achieve in MonoBehaviour world. For 1500 physics movement players like in WoW, you would definitely need DOTS or a server that does not run in Unity. Imho Unity & MonoBehaviour is still a good option. Better to release even a 500 CCU MMO with 250 CCU per instance, than never release a 1500 CCU MMO.