Numbering varies:

Code c (008/13) indicates that some issues of the series are numbered; others, published concurrently, are unnumbered.  If the numbered items are traced as series added entries in bibliographic records, field 642 (Series Numbering Example) contains the structure of the numbering that is used in the tracing.

Example:

008/13   c

130  #0  $aActualités de biochimie marine

642  ##  $av. 5$5DLC