Are you always aiming for a single persona? Or are there situations where two or more make more sense?

More than one persona can definitely make sense.  It just depends on the business.  Ideally you want to have one persona per “target audience”.  The balance is having enough to create coverage across the segment(s) that are strategically valuable to the business, and not too many that they become information overload and are not actionable.

Always aim for as many personas as you encounter, but see above for my explanation on identifying a target segment and then choosing the personas you want to go after. It's important to stay focused on key decision makers, budget holders, and influencers. Part of this may be trial and error with continuous learning as you enter sales cycles and learn more. 

There are situations where multiple personas might make sense. If you are just getting started, I'd focus on your primary persona and get that right. Does your product solve problems that multiple personas have? If so, you could extend your messaging to that persona. But don't assume the same message for your primary persona will land with your secondary persona. Do the persona research or risk throwing away marketing dollars.

I had a product several years ago and the primary persona was the sysadmin. We built some functionalty to deeply montior SQL server and decided to also sell this product to DBAs. In the initial launch, we used the same message for both personas. FAIL. Lesson learned - do the persona and messaging work.

In my experience, it's rare that a single persona would be particularly valuable. For example, in my current role, there are at least 3-4 main personas who all have very different motivations, roles, etc... 

One of the most basic examples of why you'll probably need at least two personas, at least if you're selling a B2B product, is that it's very rare that you are every selling to just one person. In most B2B scenarios, there are at least two people involved in the purchasing decision process. As you go up market and begin selling to enterprise, this number can skyrocket to 7-8 people. 

For example with my product, there are almost always at least two people involved: the economic buyer, and the user. It's rare that someone purchases our product (annual contract, >$20k ACV) without the input of their boss. 

It's important for our sales reps and the rest of the organization to understand that even if there is only one point of contact at the company, that social media manager (our main persona) is often submitting everything to their boss for their review. 

As you go up-market, there is at least two steps to making the sale: 1) selling to your user, and 2) selling to the rest of the people they need to convince. 

Most companies will have multiple personas. There are two distinctions here - user personas and buyer personas. 

If you're selling mostly to SMBs, you may only have 1-2 buyer personas (usually CEO or COO) during your sales cycle. The more you move into mid market and enterprise, the more people will be involved in the buying process and therefore you'll need to know how to sell to each persona.

For product-led growth companies with a freemium or trial product, you'll also have user personas. For SMB, the buyer persona and user personas often overlap. However, in enterprise deals the user is usually never the buyer. Your segmentation for product adoption and retention will revolve around your user persona, whereas your buyer persona sticks to the sales cycle. 

Definitely multiple persons, the number of which really depend on:

1. Number of stakeholders involved in the purchase decision (usually at least two or three; user, champion, buyer - more if it's "purchase by committee").

2. Number of distinct customer segments you are going after (note: I use a somewhat nontraditional definition of "segment").

3. Number of non-customer stakeholders (like strategic partners) also must buy in.

While there will likely be some significant overlap, the reason to consider so many different personas is the diversity of incentives each stakeholder will respond to. It's not just about customer-level value proposition, because what's in the best interest of each individual is surely to conflict with that on some level. The real challenge (fun) begins when it's time to reconcile all of them with the customer-level value prop, and with each other.