How many times do I have to beat the elite four so that I can get the uknown?

Only once.

You only have to have 6 island available I think it is (the one with lots of little islands below it) and there is a place on the island where if you push the boulders onto the right spots then 'a big rumble in the distance' is heard then you go to the small islands and different uknowns are in each cave on each island

Only o0ne.

Only once. Just solve a Strength puzzle to the south of 7 island. Then go even further south. You'll see the Tanoby Ruins and unknown are in there.

Only once dude

You have to beat it once, then follow the story (get the Ruby on One Island) and go to Seven Island. Go South and you will see a cave with lots of boulders. Use your head and think of a way to push them into holes, then Unown will appear in the ruins located on the south of the Island.

It's my first time to answer a question so here it goes,
You only need to defeat the Elite Four one time,
to get an unown you must go to Seven Island
When you get their you must turn left and
Then go down until you get to the ruins.
and Surf to the little islands go in
The caves and you find unowns. (note:you could find a shiny unoun colour blue)
I hope I helped!!!!

Only once