As of 4D v19 R5, it's possible to use the .getCount() function to return the number of entities in a Dataclass.
All Dataclasses in a 4D application are available as a property of the (ds) datastore.

Example:

var $ds : 4D.DataStoreImplementation var $countPersons; $countContacts: Integer $ds:=Open datastore(New object("hostname"; "www.myserver.com"); "myDS") $countPersons:=$ds.Persons.getCount()   // remote datastore $countContacts:=ds.Contacts.getCount()   // main datastore

Note:
If this function is used within a transaction, entities created during this transaction will be taken into account.