I believe in the movie itself we are only able to see perhaps five onscreen at the same time, and I'm also pretty sure there is a point after two have been destroyed during the battle where we still see at least three together on screen. So is five the best official number available?
It depends which source you look at, but somewhere between five and twelve.
According to the official novelisation (by Donald Glut), there were at least 12.
Through the blizzard, the small dots seemed to be advancing at a slow yet steady pace, churning up clouds of snow as they moved toward the Rebel base.
The officer raised his electrobinoculars and focused on the approaching objects. There must have been a dozen of them resolutely advancing through the snow, looking like creatures out of some uncharted past. But they were machines, each of them stalking like enormous ungulates on four jointed legs.
The single largest number seen on screen in ESB is the group of five making the direct assault on the base. As far as I can tell, two more walkers (one carrying the AT-AT commander) approach the shield generator from a different angle.
Taking into account the one destroyed by Luke's speeder, the one destroyed by Luke's grenade and a deleted scene in which one of the walkers was destroyed by blaster fire (see below) that means that there were at least five and possibly as many as seven in the battle.
The Star Wars Databank, considered a canon source of information about the Star Wars universe, contains this still image of the Battle of Hoth with six walkers visible.
This Galaxy of Adventures short film shows seven walkers in loose formation. Six are immediately visible and one is hidden behind the second from the left and is seen shortly afterward.