Hello,
We’re aware of this issue and will explain. The number is showing orphaned events, which is actually ‘correct’ and is now showing due to a fix we introduced in 5.9.7. The side-effect is uncovers the fact that orphaned events can exist more obviously.
To correct that number, visit Events > Settings > Admin Tools and then click the ‘Remove Orphaned Events’ button. You’ll notice that in the description the number should correspond to the difference in that menu number.
The other issue is orphaned events. These happen when the event isn’t properly deleted. The problem is it does not happen all the time, and we have been unable to reproduce the problem due to its rarity. If you ever notice that the number is off whilst deleting events or performing an action, we’d appreciate knowing the steps taken. If we can reproduce the issue, fixing the problem becomes a possibility.