In my opinion, no single design is apt to be optimal for everyone. (Don Norman)

And that’s why we have personas. Now, without them, there are several defaults. Don touches on one of these – trying to be all things to all people. Other possibilities are designing for yourself or designing something so bland and noncommittal that you’re really not designing for anyone at all.

So, how to avoid all that? Right off the bat, you know that you can’t create a persona for every possible user or customer out there. You do have to narrow them down. 

But how many do you need? Personally, I generally let the data speak for itself. Now, I can guarantee that there will definitely be more than one. Chances are, though, that there won’t be more than 5 or 6. And, even then, you can typically divide your personas into primary ones and secondary ones. (Just make sure you focus on those primaries!)

The important thing is that you are designing for someone specific. But, if the personas have been designed properly – with believable stories, and real names, and realistic photos – it will be hard for the team to not do so. It's just the way the human mind works.

Once you have your personas, you now have several ways to do deal with them. You might, for example, come up with multiple designs. Or, you might want to design for one of your primary users, but make sure that other personas can still get to what they need (just don’t throw it in the primary users' faces). You might have levels – like in video games. Finally, you might want to simply make a business decision and focus on your primary personas (probably your main source of revenue anyway) and let the secondary ones go.

That said, I have also seen designs that do cover multiple audiences quite well. I’m thinking, in particular, of long sales pages, where the user simply keeps scrolling until they're sold. For the detailed-oriented type, they’ll be scrolling through the whole thing. For the shoot-from-the-hip type, it’s a few bullet points and a CTA. For the socially-oriented, reviews and testimonials are often what they’re looking for. For the hard-sell, show-me-the-money types, it’s usually showing your low fees, or your high interest rate, or how your price compares to other vendors.

Do note, though, that this is definitely the exception. The rule is exactly what Don says.