IF you have:
Count ( TableB::NeverEmptyField )
Defined as a calculation field in TableA will produce the count of related records in TableB. In place of "neverEmptyfield" you may refer to any field that stores some kind of value in every record. An auto-entered serial number field or a foreign key field thus make good choices for this.
Where sCount is a "count of" summary field that counts a never Empty Field will, if placed on a tableA layout or referred to in a calculation evaluating from the context of TableA, also return the same count.