As it can be displayed in various forms according to it's use case here, it can be displayed either 99+ or 999+ like in your case preferred as 9+.
Depending on the context given above, mainly two factors seem relatable here;
The first and the foremost one is, there's a limited space for the digits on the display to show the exact number where it's enourmously big like thousands/millions.
Secondly, it's mostly preference depending on the context (of the environment, the notification type, personas, etc.) whether it's less or more than the number represented as an ideally possible number. So that, it's not matter to display exact number, other than informing users as it's specifically more than a bunch of them.
For making a brief more clearer, consider a mailing mobile app for instance. Let's say users expected to have ideally around a couple of new e-mails to hundreds of them in their inboxes. Besides the mobile app icon has a specific width and height so that it can to say, only hold 4 digits as a badge for notifications on top of it.
In this case, setting the notifications badge up to
999+ is the optimal choice for both providing an icon on mobile that seems looking good and informing users enough to let them it's time to take a look at it since it's more than ideally expected.
Best practice: Besides an example use case above, it's also beneficial to know how it's preferred to be displayed. Again let's consider a case that ideal users expected to have a couple of notifications, and we're looking for a better solution to display notifications when it has reached 5, 6, 7, etc digits. Like if it's making no sense to let users know about the exact number of this notification type, it's preferred to be shown as