Grouping people's traits into personas is not "a step before", it is rather "a step after" the initial interviews.
As you probably already know, the persona should be probable, believable, consistent, have boundaries and so on... That means, persona is a representation of people who share similiar:
The main problems with that are (mainly) two:
- People do not directly speak about their needs
- People usually mix the roles they act as
So, they would (for example) tell you what they would like to see or do but not why they need that representation of information or action. So, before you can recognize such needs as the same, you should first recognize the needs.
They would also, easily mix the roles they act as, because they mix it as easily in a real life. So (for example), they would tell you about their use of your "job portal" as a person who seeks for a job and a moment later, they would tell you how they use it as a person who wants to post a job ad. So again, it's your job to separate those behaviour into a separate personas with separate goals...
If you really, I mean really, encounter the 20 different people with 20 different motives, then you should refer to the vision of your product. Whom are you building it for? With limited resources you can not build a product for everyone, their motives may be contrary also. Your target and your vision should guide you when creating the personas.
Edit, case study type example:
Lets assume that people bruggle (a fictional verb). Our vision is to create a product which will help people who bruggle on their own.
We do research and initial interviews. It looks like most of the people can't bruggle alone, it is too complicated and they need professional help.
So we change the vision and decide to support such bruggler-helper relation. We adjust the group and continue the interviews. There are some small companies which help to bruggle and ex-professionals (or professionals) who do it off hours.
The interviewed people tend to mix their roles. One minute they are speaking about how they work in company, and the next minute they are speaking about how are they helping off-hours. Both roles have a common goal (or set of goals): to help the bruggler. It looks like a one-persona-situation. If you ask about their preferences, thinking, principles, you will probably get an answer related to one of the roles they are currently thinking about. It is difficult to realize there are two different mindsets in play, because interviewed person does not make such distinction (but when we do realize it, it becomes obvious...)
After interviews, it becomes clear: both professionals and companies, in order to generate enough income, need to work with multiple brugglers. One bruggle-helper may use our product but not all of his brugglers will. Since our product will have a significant impact on them and since we can not afford to miss such opportunity - we will be forced to create a persona for a bruggler who doesn't use our product but is in the circle of its influence.
And it goes on and on... Of course, not all of the personas are primary, but there are often a very strong reasons for creating one. I can not call it "one persona per motivation".