How many on-campus interviews (for a tenure track math position) do you typically need to get one offer? I was wondering, on average, how many on-campus interviews one needs to get one offer? For those who landed a tenure-tack job, could you please share your information?
I expect that conventions for interviewing vary greatly between fields, countries, etc.  I'll answer based on my experience with mathematics in the U.S.


  I was wondering, on average, how many on-campus interviews one needs to get one offer?


At an individual level, you can't say for sure.  Not only is the variance high, but there are also systematic differences between candidates even aside from overall ability: some people come across much better on paper than they do in person, while others interview really well.  There's no way to figure out in advance how many on-campus interviews you will need to get an offer.

However, you can ask how many candidates are interviewed for a typical opening, and that will tell you the average number of interviews needed per accepted offer (just not in a way that says much about individual candidates).  That varies between departments, depending on how decisive they are, how energetic they feel about interviewing, and how much funding they have for the search.  I'd estimate that interviewing three candidates is pretty typical, although I don't have precise statistics.  It's not likely to go below two except in unusual circumstances, and interviewing six people for one position sounds rather high to me (although not unheard of).

So this means that, on average, you won't get most of the positions you interview for, but you have a realistic chance at them.