"A Google support page states that “up to 100 people with view, edit, or comment permissions can work on a Google Docs, Sheets, or Slides file at the same time” and has a section devoted to troubleshooting files that become unresponsive after being shared with many people, recognizing the common pitfall."
That's a big number, but it's not as big as I'd expect from something from Google. I'd assume the average number of people working on any Sheet is 1 or 2, but I'm surprised that haven't been able to scale it up to 1000 or more by now.