This works the other way too though. I've seen some parents use the 5 minutes warning and then pack the kids up after only 2 or 3 minutes. The kids don't know the difference and it allows the "warning" to be consistent even when the timing can't be for whatever reason.
And yes, 10 more times down the slide can easily turn into 9 times in a row and holding on to that 10th time for as long as possible.
Better to use accurate language like "5 more moments" or "time to wrap up" or "one last ride", to avoid messing up your child's budding sense of chronology and numeracy.
And yes, 10 more times down the slide can easily turn into 9 times in a row and holding on to that 10th time for as long as possible.