There really isn't a reliable number for this. Some people only have one or two interviews and get an acceptance out of it. Others have 9 interviews and get no acceptances (though this is very uncommon). If they invited you to an interview, it means that they think you would be a good addition to their school. Whether or not you get accepted after interviewing is based largely on whether or not you presented yourself well and seemed not awkward, arrogant, rude, disinterested, or otherwise unappealing in your interview.
The number of interview invitations you get is based on how good you look in your primary and secondaries. The number of post-interview acceptances you get is based largely on how well you interview. Since these two things are very different from each other, we can't say with any sort of certainty how many interviews a person would need to feel confident that they'll get an acceptance.