Arbitrary Limits are Bad
Arbitrary limits are rarely a good thing. I am sure there is a limit somewhere (maybe 256 tabs?) but as long as it is far out of normal usage, most users never know and will treat it as "infinite", which actually makes for a better user experience.
Having to manage "only up to 4" or "only up to 8" or some similar small number of documents is very limiting for the user. For example, classic WordPerfect 5.1 allowed easy simultaneous access to 2 documents. Want 3? Out of luck. Today's word processors - no set limit obvious to the user.
The same issue crops up in many different places:
- Maximum rows or columns in a spreadsheet
- Maximum size of a free-form text field (always best to make it so large that the typical user doesn't even know there is a limit)
- Maximum simultaneous programs running in an operating system - unfortunately, this bumps into serious issues with memory and other resources that can't be avoided - but the general idea is to not impose an arbitrary limit.
And then we get to who actually uses so many tabs in a browser:
Obviously, power users do this. Open news, weather, Google, StackExchange, company database, plus several tabs of actual work.
Casual Users + Poor Web Design