If 20 is too few, then 10 is too few. 50 is definitely too many. So 30 or 40. If 40, then I say any writing should be allowed, ie plays, monographs, poetry etc as well as books of fiction and non-fiction.
Personally, I still like 20 though I see the problem of little overlap with this number.
We could consider a large number, like 50 and only rank the Top 10. Isn't that how we did the Top Films of the Decade?