Personally my pekin hen would sit on 18 perfectly but It depends on both the duck and the eggs. For example our welsummer tried to sit on 6 Orpington eggs but only 3 hatched and to never even began incubation simply because she couldn't cover them! I'd generally prefer my broody hatched in the small size 3-6 each so that once they were larger the hens could still relatively manage them with little risk.Then again I free range constantly! So really it depends on how dedicated your bird is,how gentle (at risk of crushing eggs), and what you think the both of you can handle.(I only let one of my smaller hens raise in trios cause she was so small she couldn't defend them all against my other hens in larger numbers) I'd stop at 20 though, since I'd doubt she'll hatch them all, plus other hens may add eggs when she's not watching so I like to keep the numbers down so I can keep watch ;D