Few things change the shape of an event as quietly as plus ones. You invite twelve people you know, allow plus ones, and suddenly half the room is strangers to you. Sometimes that is exactly what you want. Sometimes it is the opposite of what you wanted. The decision is worth making on purpose rather than by default.
Ask what kind of night it is
Plus ones suit open, mixing events: a big birthday, a housewarming, a summer party where the goal is a full warm room. They suit intimate events badly: a dinner for close friends, a small gathering where the point is the specific people, a night built on everyone already knowing each other. Before you decide, picture the room. Is it better with a few unknown faces, or is the guest list the whole point?
Set it once and let the form ask
When you create the event you can allow plus ones and set a limit, one each or more. Guests then declare how many they are bringing when they RSVP, and your headcount includes them. You do not have to field the can I bring someone text and make a ruling each time. The setting answers it for everyone, consistently, which is fairer than saying yes to the friend who asked and feeling unable to say no to the next one.
Headcount honesty
The reason plus ones matter for planning is that they hide in your numbers. Twenty guests who can each bring one is a possible forty. For anything with catering, seating, or a capacity limit, that gap is the difference between comfortable and chaos. If you allow them, plan for the higher number, not the lower one. If you cannot plan for the higher number, that is your answer about whether to allow them.
When to say no, kindly
It is completely fine to not allow plus ones. The cleanest way to do it is to not make it a thing you announce defensively. The event simply does not have the option, and if someone asks, "keeping it to the core crew this time" is a complete and friendly answer. You are not rejecting their friend. You are choosing the size of your own night.
The default that ages well
For most casual events, one plus one each is a good middle setting. It lets people bring a partner or a friend without opening the door to an unpredictable crowd. Reserve no plus ones for the genuinely intimate, and open plus ones for the genuinely big. Match the rule to the room and you will rarely get it wrong.