It depends on how long the interview process takes and for what type of job. Generally, three rounds are enough to get an idea about a potential candidate’s skills and suitability for the role.
Anything in excess of 3 interviews is draggy
Anything more than that could be seen as excessive or unnecessary from both sides – too much time spent by hiring managers interviewing candidates who may not be suitable, and too many interviews taking up valuable time in a candidate's day-to-day life.
But specialised positions really need in-depth, consecutive interviews. 3 rounds or more.
However, if it is absolutely necessary due to specific requirements or qualifications needed for certain roles then four or even five rounds might be justified - but six should really only happen under exceptional circumstances, such as when dealing with highly specialized positions where there is no room for error.
So how many interviews is really too many?
In any case seven (or higher) would likely indicate something wrong with either the recruitment process itself or selection criteria being used which needs addressing before further applicants can go through it again!