The entry is optional: the MRtrix documentation lists only 'file' and 'datatype' as required header keys, and explicitly notes that the value may not reflect the number of streamlines actually stored, e.g., when a command was terminated prematurely. The value is therefore used as a hint only, never to decide how much data to read.
parse.mrtrix.count(header)numeric, the stored count, or NA if it is absent or not parseable.
named list, the parsed header.