" Magic Room suppresses the effect of
all items held by the Pokémon on the
field. This effect lasts for five
turns. Similar to Trick Room and
Wonder Room, using Magic Room while it
is already in effect will end it
immediately. Magic Room will prohibit
the use of items through Natural Gift
and Fling; however, Magic Room will
not prevent the exchange of items
through Trick. Magic Room will not
suppress the effect of Berries if they
were consumed through Pluck or Bug
Bite. Items return to their normal
function when Magic Room ends, as
though the move was never in effect. A
Pokémon locked to a move by the effect
of Choice Band, Choice Specs, or
Choice Scarf will remain locked to
that move after Magic Room ends, but
will be able to switch moves freely
during it. Magic Room is in the -7
priority bracket and therefore will
always go last, except against slower
foes also using an attack in the same bracket. " - Bulbapedia
So to Clarify, You have 5 turns.
Switch, you now have 4 turns.
Dragon dance, 3 left
Dragon dance, 2 left
Dragon dance, 1 left
And now, you have to use whatever move you want to use during the rest of the battle, so I'm going to take a guess on what you're going to let your last turn look like.
Dragon claw, Magic Room ends, you're locked into Dragon Claw.
So you would have the oppurtunity to set up 3 Dragon dances.