In English, the plural of sheep is also sheep. Sheeps is not a word, so Duolingo would mark you wrong for that.
Do you pronounce the 'n' at the end of hebben and schapen? Is it correct to do either? I've heard it said both ways on various websites but I'm now confused.
I don't pronounce the n at the end of verbs and plurals, I don't think many people do. It sounds a bit snobby.
I actually always pronounce the 'n' in words like these. Not very clearly, but I definitely pronounce them.