Minimum number of items in the Product Backlog for an Affinity estimate

Affinity estimation is a fast estimation technique usually used for release planning. You have a product backlog full of items (or a list of the most important items) and you want to get an airplane view of how many sprints the team will need to build them all. Knowing the team's velocity and sprint length, the goal is to estimate the total number of story points for all of the items together. Then, the total is divided by the team's velocity to get a number of sprints.

Of course, like any estimate, it's an approximation. It's just to get a sense of the effort. The team might build the items as estimated, or they might not (for various reasons that were plenty discussed in other questions on pm.SE).

With the affinity estimation technique, the team groups items into buckets (i.e. the sizes of the story points) based on their similarities in complexity and size (that's what affinity means, similarities, shared characteristics). It's faster than, say, planning poker, because people discuss an item if they don't like the bucket in which it was placed, and it has the advantage that you keep an eye on all the stories while you estimate and can think about one story in relation with the other stories in the same bucket. With planning poker you can forget about other stories since you estimate one by one, so your focus is mostly on the current item.

Because it's a fast technique and usually used for larger releases, it's used with a larger number of product backlog items.

With that out of the way, the question is now: what constitutes a large number of product backlog items? 5, 10, 20, 50, 100? As Daniel pointed out in his comment, this is context dependent. The answer is in the material you are using for the exam, or the material that you were taught. Whoever wrote the material and the test question is also the one that established the number.

If you ask me, the question is really dumb. If the answer is 20 (which I'm not saying it is) and the Product Owner shows up with 19 product backlog items, then what?

Check your material, or ask your teacher/trainer/coach.