I currently run around 150 perspective sessions on a single gateway simultaneously. Many have embedded views, flex repeaters with ~100 instances, etc. Really it comes down to your hardware.
Of course, you can overdo it with embedded view nesting, but Ignition can handle a lot with good hardware. I think right now, I'm running 12GB and 4 processors, which will be increased soon.
Note that some people break up a gateway into a frontend and backend gateway split to handle heavier loads to bolster performance.
This thread has some insight