The simple answers: yes.

The complicated answer: depends.

The more complicated answer: depends first and foremost on what is your method and research question, and many times on your discipline. For instance, in ethnography the answers is typically no. You'll need to have observations, and at times some documentation. In discourse studies, sometimes yes and sometimes no. An typically even when you have interviews it is not the "in depth" aspect that you are after. In grounded theory, it varies, but often it is possible. In narrative research (not of the highly discursive type wherein the former note applies, e.g., Alexandra Gerogakopoulou's studies or Michael Bamberg's studies) most of the time the answer is absolutely yes (e.g., Ruthellen Josselson's studies, Mark Freeman's studies etc..). If you're interested in "narrative realities" (Gubrium & Holstein, 2009), however, you might wish to have more information, though the narrative will be you main source. In phenomenology, of most sorts, the interviews will be sufficient.

In qualitative studies (case study research, grounded theory, ethnography, etc) the basic issue is to gain so called data saturation. You reach it by gaining as much information as possible and you collect data until new information do not bring any new knowledge to what you are study. Therefore, you use diverse methods of gathering data, and multiple sources usually mean different standpoints, views, etc. It is best to combine some hard information (from documents, for example), observation, interviews, active participation and then integrate it into a single theory.

Cheers!

Tomasz Ingram 

A qualitative study can rely exclusively on interviews, the question is how many interviews are needed. Check this article for instance:

http://journals.sagepub.com/doi/abs/10.1177/1525822X05279903

@Antonio Carvalho - you are obviously right - It can :) I forgot to mention it.

Thanks for that, very helpful.

From my reading, studies that rely solely on qualitative interviews are the most common kind of qualitative research. And 12 interviews certainly could be enough, if you have followed standard approaches to saturation.

The following papers could further help: 

• Alshenqeeti, H. (2014). Interviewing as a Data Collection Method: A Critical Review. English Linguistics Research, 3, 1, pp. 39-45.
• Doody, O. and Noonan, M. (2013). Preparing and conducting interviews to collect data. Nurse researcher, 20, 5, pp. 28-32.
• Hennink, M. M., Kaiser, B. N. and Marconi, V. C. (2016). Code Saturation Versus Meaning Saturation: How Many Interviews Are Enough? Qualitative Health Research, 27, 4, pp. 591-608.
• Marvasti, A. (2010). Interviews and Interviewing. International Encyclopaedia of Education, 3, 4, pp. 424-429. 

The simple answers: yes.

The complicated answer: depends.

The more complicated answer: depends first and foremost on what is your method and research question, and many times on your discipline. For instance, in ethnography the answers is typically no. You'll need to have observations, and at times some documentation. In discourse studies, sometimes yes and sometimes no. An typically even when you have interviews it is not the "in depth" aspect that you are after. In grounded theory, it varies, but often it is possible. In narrative research (not of the highly discursive type wherein the former note applies, e.g., Alexandra Gerogakopoulou's studies or Michael Bamberg's studies) most of the time the answer is absolutely yes (e.g., Ruthellen Josselson's studies, Mark Freeman's studies etc..). If you're interested in "narrative realities" (Gubrium & Holstein, 2009), however, you might wish to have more information, though the narrative will be you main source. In phenomenology, of most sorts, the interviews will be sufficient.

Depending on the level of studies you were undertaking, 12 interviews could be enough for first degree research project students, and to some limited extent to Masters degree dissertation students provided that the process of data saturation has been reached. I feel that those Masters degree students and Higher Degrees candidates who are searching for the academic shine and production of a research output document that gains unanimous academic acceptance in the scholarly community, they complement data from in-depth interviews with data from observations, documents, personal accounts, memoirs and active participation on the part of the researcher, where necessary.