Many, many times, but an objective count is doomed to failure. Not only is the number in the TV series alone practically countless, but all the various spin-off media not only complicate matters by increasing the number, but also lead us to the question of what counts as 'canon', not an easy one to answer is Doctor Who. It could even be argued that what counts as 'saving' Earth is opinion-based (people vary about the morality of deposing Harriet Jones in "The Christmas Invasion", for example). What we do know, however, is that Earth is his favourite planet. In, "Logopolis", for example, he describes it as his 'home away from home'.