This is from memory, so I may be wrong, but the directive is if a ruck is formed, both your feet need to be behind the ball.
If there is no ruck at that point (i.e. maybe the defenders disengage and fan out), one foot.
Pfitzy's pretty much spot on. This is from this years Game Management Guidelines
"At a tackle, a player can pick up the ball as long as one foot is level with or behind the ball. At a ruck, a
player can pick up the ball as long as both feet are level with or behind the ball. If a player is still bound
with an opponent, then they cannot pick up the ball."