Hello,
There is no generic DICOM tag that contains the number of DICOM instances that are contained in a given series, and there is no generic tag that contains the index of a DICOM instance inside its parent series. It is thus in general impossible to know whether a series that is stored by Orthanc is complete: This is the semantics of the "Unknown" series status.
However, for some types of images (such as cardiac MRI), the DICOM modules might specify a tag that contains this information. If such a tag is available for a series, Orthanc will either report a "Complete" status (if all the instances have been received), or "Incomplete" (if some instances are still missing). The "Inconsistent" code might also be reported, if there is an error inside the numbering of the instances.
HTH,
Sébastien-