Welcome to the weird and wonderful world of Vivaldi's multiple-instrument concertos!
The term 'con molti istromenti' (Vivaldi's own) refers not so much to the literal number of instruments involved as to their variety and unusual deployment. For an Italian concerto of the period to use wind instruments at all would be unusual; Vivaldi combines recorders, oboes, chalumeaux, horns and bassoons alongside a wide-ranging complement of strings, itself including viole all'inglese and the even more obscure violins 'in tromba marina'.
The composer's opportunity to write such works arose largely from the talented pool of girls available at the Pietà orphanage in Venice. These expert musicians would turn their hands to whatever Vivaldi could possibly desire and the resultant scoring is colourful (sometimes outrageously so) and inexhaustibly inventive.