you need all badges but only surf is needed. However,for the cave near the end you need strength
First you need the first badge then you need the HM Flash which is given by the oak aide on route 2 lastly go inside rock tunnel and choose the Pokemon that has flash then choose flash the cave will be lighted up more!
my but
You need all eight badges and you have to have beaten the Elite Four in order to capture Rayquaza.
4
At least 2 badges are needed to trade from leafgreen to firered.
The gym leaders have the badges all you need to do is defeat them.
you need to beat the 8 gym leaders
U need 7 badges to get it
You need 3 badges to breed Pokemon.
8 badges
Access to the elite four at the indigo plateau but first you need to go through victory road good luck
you need cut, fly, flash, waterfall, surf, rock smash, and strength
4.
4.
six or seven
once you see the building you will need certin gym badges to open different doors.I also think the eliet 4 is in that buiding.