Nine people said yes in the WhatsApp group. Four of them were reactions to a message, which is not the same as a reply, and one of them, Tom, has apparently brought his partner and two cousins without ever being asked how many seats that needs. Marta reads the thread at midnight and has no idea what the real number is, only that it feels like more than the room holds.
An event RSVP website replaces that thread with a single page guests reply to. Nobody types their answer to the group anymore. They tap yes on the invitation, name who is coming with them inside whatever limit you set, and the reply lands against their name on your list. By the time you close the list, the count is a fact rather than a feeling.