Normally, we use Active Directory security groups for managing permissions in SharePointsites. The users in this Active Directory group are synchronized through the user profile Service Applications to SharePoint.
If your Active Directory group is updated with the users (added or removed), normally it should be synchronized to SharePoint sites, wherever these security groups are already added.
I have noticed many times, although the users are added/removed from the Activity Directory groups, still the # of the users are showing incorrect in the Target Audience in SharePoint sites.
Look at the screenshot given below.
Edit the WebPart -> Advanced -> Click Browse -> Search Active Directory Group name -> Verify the Total Members
If you are seeing incorrect # of members for your Active Directory group, then the solution is run in the User Profile Full Synchronization.
Note- If you have large # of user profiles, try to run the User Profile Full Synchronization in the off business hours to avoid performance issue.
Hope, it helps.