Overview

You need to know how many people are attending or RSVP'ing to meetings. People are able to add events to their outlook calendar. However, you are unable to see how many people have responded to go to the meeting despite having "require attendee response" enabled.

Solution

Permanent Solution:

Follow this article “Manage large lists and libraries” to resolve the issue by using the modern experience, adding indexes, or editing the list view.

Temporary Workaround:

Checking the List View Threshold

• On the Site Contents page, click the 3-dot menu of the Event Attendees list.
• Select Settings.
• Check if there is a notification indicating that the "list exceeds the list view threshold."
• “Event Attendees” list view cannot exceed view threshold of 5000
• Delete older list entries to ensure the list is under 5000 so the event attendees now show in a subsite