She'll try to hatch whatever you give her, but since duck eggs are on the large side and she's probably fairly small I wouldn't try to put too many under her. Maybe 3-5, you'll have to see how they fit. The other thing to keep in mind is duck eggs take longer to hatch so if she's been broody awhile she may (or may not) give up on them before they're done. I've hatched duck eggs under hens before so I know it works, you just don't want to put them under a hen that's already been broody for awhile unless you know she's one of the ones that will sit for ages. Good luck!
If you have any water nearby be sure to watch when the hen takes the ducklings out for a walk and they decide to go for a swim....my poor hen went bonkers, I think it may have scarred her for life.
She's only been broody for about 24-48 hours now. I went ahead an put some of the duck eggs under her. 6 seemed to fit just fine but when I put 7 under her she couldn't cover them all completely so I just let her have half a dozen. I hope she stays with it and hatches some ducklings!