You only have six digits but the like is you only get three bites at the apple and it becomes a useless brick. Not even the FBI could beat those odds
When I forgot my iPhone password I was given multiple "3 bite" tries, beginning a few minutes later and working its way up to more minutes than I cared to deal with (at which point I wiped my phone). Has that changed?
But as I said, the number of digits may be trivialized if a user opts for a pretty pattern, as is possible on an iPhone.
If I've never mentioned it, I learned during my attempts to guess my password that if your iPhone is not up to date, you can regain its use by restoring from a backup without knowing its password.