There's really no set answer to that.
With bikes, everything has been done on anything.
Around the world on a high-wheeler - sure.
Coast-to-coast(US) on a unicycle, naturally.
I think there's one guy who set out to ride all mainland states on a BMX, backwards.
Touring on a 5 speed will not even stretch the oddity barrier.
It's not only about the number of gears, it's also about which gears they are.
I could easily see myself doing a "casual" tour in flattish country with only five gears.
IMO, walking a bike uphill is more boring than coasting downhill, so I'd set the range fairly low and sacrifice the powered descents immediately.
For a European tour, there are several river valley routes with very little elevation gain wher this is likely to work fine.
Me, I wouldn't want it if I planned to hit the Alps.
But that doesn't mean it can't be done.
And there's no natural law preventing someone from even enjoying it.