1
Only one.
1 unless you get first in Pokemon lottery in the goldenrod radio tower
Well you can get a action replay code and use it to get 999 Master Balls
You can get 2 Master Balls in both versions.
you can get 1 master ball
1
i masterball
one
You can clone a Pokemon with the master ball or trade it to a friend and keep starting over until you're satisfied with the number of master balls.
Attach your master ball to ONE POKEMON, next CLONE that Pokemon for how many master balls you want. Next detach them, and release all the clones (if you want)
Duplicate Pokemon with a masterball attached to them.
I think you can get only one.