Depends entirely on how many they think they need to give them a good chance of finding a decent candidate. I'd be happy to do 5 or 6, given the choice, as long as I thought that at least 3 were good enough on paper to do the job. If I had doubts, I might interview more.
On 2-stage, would depend on the stages, tbh. If stage 1 is a 15-minute phone interview, I might sift up to 15 or so for that part and then choose 5 or 6 to interview. If stage 1 is psychometric tests marked by computer, you can have far more people doing that and then choose. If stage 1 is the full-blown interview and stage 2 is a final interview with The Big Boss, then I might interview 6 and select 2 or 3 max for the final stage.
I vaguely agree with jkkplu- but it will depend on the size of the company, and whether you're interviewing with a specific talent department, or going straight to line.
I'm part central talent department in a multinational, and will look through up to 300 applications - phone interview 30/40 for 15 minutes - invite most to psychometrics - first stage interview approximately 15 (with 5 or so in 'reserve' if they were any good) and then put forward a shortlist of 4 or 5.
But we'll interview as many as we need to, to get the right person.