My science teacher asked us this, and nobody in my class knew. We guessed it would be three, but with no evidence.
(Edit: The asker has not returned to clarify if they meant leaving the Earth-Moon system, or re-entry from orbit. As it has been a few days, and this question has received lots of attention, I have edited the title to reflect the common interpretation of 'leave orbit', and the content of most answers. Note that answers below also address the possibility it refers to re-entry. It would be a shame to close this, and so I think this minor modicum of interpretation is justified.)
