The limit of 100,000 objects is a rough estimation. The actual limit if you're using SQL Express (included/installed by default if you don't have a full SQL Server instance selected during install) is 10GB.
Generally, we refer to "objects synchronized" in this context by going to the AAD Connect Synchronization Service Manager and clicking Tools -> Statistics. In the window that pops up...
You want to add up the "Objects" values for any on-prem AD connectors. Do not include the AAD (x.onmicrosoft.com - AAD) count.