Documents:
If you and another collaborator are editing the same document at the same time, a box with the name of the collaborator appears at the top of the screen.
10 people can edit a document at the same time. Once this limit has been reached, others will be able to view, not edit the document. You can share a document with up to 200 people.
Spreadsheets:
If multiple people are editing or viewing your spreadsheet at the same time, you'll see their names listed at the top. Click the arrow to the right of the names to open a tab where you can chat with other viewers and collaborators about changes being made to the spreadsheet.
A total of 50 people can edit a spreadsheet simultaneously. You can share a spreadsheet with 200 collaborators and/or viewers.
Presentations:
If other people are editing a presentation simultaneously with you, you'll see their names at the top of the page. You'll also be able to chat with other viewers and collaborators in View presentation mode (available in the upper-right corner of the edit page).
10 people can edit a presentation at the same time. However, you can share a presentation with up to 200 people (whether you add collaborators or viewers, the total cannot exceed 200 people).