When you try to synchronize more than 20,000 objects to cloud, you receive the following error message:
Event Type: Error
Event Source: Directory Synchronization
Event Category: None
Event ID: 102
An error occurred. Error Code: 16. Error Description: Synchronization has been stopped. The company has exceeded the number of objects that can be synchronized. Contact Technical Support and ask for an increase in your company's quota. Tracking ID: <Tracking ID> Server Name: <Server Name> Contact Microsoft Online Services Technical Support.
By default, the Office 365 Directory Service (DS) quota for each Office 365 customer is 20,000 objects (or 10,000 objects for accounts that were created before October 5, 2011). The on-premises Active Directory might contain more than the number of objects that are allowed to synchronize. These objects include all users, all groups, and all contact objects. Therefore, Directory Synchronization (DirSync) can synchronize only the first 20,000 objects (or 10,000 objects for accounts that were created before October 5, 2011). After the quota limitation is reached, Directory Synchronization cannot add more objects until the number of objects drops below the quota limitation, or the quota limitation is increased.
Note: To increase the number of objects that can be synchronized, you must contact Office 365 technical support.
How to request a Directory Service quota increase in Office 365 when you receive the "This company has exceeded the number of objects that can be synchronized" error
Office 365 Exchange Online post deployment