UX designers use personas to bring their understanding of users into their design work. A persona is an amalgam of people seen in design research, an amalgam of a type of potential user seen in the field, fictionalized and abstracted enough that the designers don’t get too hung up on the irrelevant idiosyncrasies of particular individuals.
But an oversimplification nullifies the purpose of using personas.
“Everything should be made as simple as possible, but not simpler” — Albert Einstein
One part of that practice we don’t talk about enough is how to build out a set of personas to describe the shape of the user population when we have found varying types of people. As Einstein would have it, we aim to limit ourselves to as few personas as possible — but no fewer than we need. So how do we know how many that is?
Personas are like tentpoles under a circus big top tent.
Users stand under the tent. Designers want to cover as many people as possible with as few tentpoles as possible, giving people plenty of headroom; the further from the pole people stand, the less design which targets that tentpole — that persona — will serve them. Generally a designer building personas places those tentpoles somewhere in the middle of clusters of people; no one person is just like the persona, but enough people are similar enough that targeting the design at the persona addresses all of those people. Often observing users makes this easy; one can see a few clear clusters of people.
The structure of the user population requires some art to address.
There can be too much variation to represent with a single persona.
It may happen that it makes no sense to place your persona in the middle — that leaves too many people poorly represented — but if one plants a tentpole near one end of that cluster and another tentpole at the other end, it covers the whole range. Two personas may thus represent not distinct types in the population but rather tendencies among an unstructured space of users or a range of people who differ across many axes.
The reverse can happen: one persona can address a few different roles.
Maybe a real estate broker is a good persona to represent a professional who handles a lot of paperwork on the go, and can also stand in for a salesperson, an insurance inspector, et cetera. This looseness in the relationship between how personas are defined and the roles people fill reflects how designers should not define personas in terms of the tasks people do but look a layer deeper to the goals they pursue: what they try to accomplish, and how they think about it.
And there is certainly a benefit to keeping the number of personas low as they have to be constantly iterated and updated. If you have an army of them, any tiny change will result in hours of work and I cannot say that this work is necessary.
So how many personas can represent users in a domain?
There is no magic number; it depends on the depth of user research and that domain. In a consumer space with a broad range of people, it may turn out that people have a range of widely divergent goals, requiring several personas, but in many cases even a big population of people have such similar fundamental needs that only one or two well-conceived personas will serve. Alan Cooper, the inventor of personas, talked about how the creator of the rollaboard suitcase was a pilot who was thinking about the needs of people on flight crew, but that single design persona produced a solution that suits most people who fly.
How many personas do you need to design your product? As few as possible— but no fewer.