You might not be able to import all users from the Active Directory source using LDAP sync. By default, in Active Directory, LDAP search returns only 1,000 objects. This issue is caused by a default setting in Active Directory that limits LDAP searches to 1,000 results.
To work around this issue, you can use the Ntdsutil.exe tool to modify the value to expand or limit the search, depending on the size of the Active Directory tree.
For more information, see the Microsoft support article on How to view and set LDAP policy in Active Directory by using Ntdsutil.exe.