45 pearls plus clasp...if the necklace is 18" then the pearls are about 10mm as Douglas said.
To me they look like they may be imitation pearls.
Do this:
Rub two of the pearls together after cleaning them to remove surface grime. If it feels gritty it's likely real cultured pearls; if the pearl glides easily and smoothly it's likely imitation. Edited to add: I have stopped recommending rubbing pearls against teeth (“Tooth test”) Teeth are harder than pearls and can scratch them.
Also, see if the metal clasp has any marks indicating the metal -- 10K, 14K, 585, 750 for gold, 925 (silver or silver with gold plating), 14KGF or 10KGF (gold filled). There may also be a brand mark.
See if there are any places the coating is peeling off or already peeled off revealing the bead inside.
Examin around the drill holes for any swirls of extra coating, which may be present with some imitations.