The key of C# major has seven sharps in it. So includes not only a B# but also an E#

However, more often than not this key is notated in the enharmonic equivalent of Db (I suppose five flats are easier to get your head around than seven sharps)

Whichever, it's a bugger of a key to play in (that's why capos were invented!). Goodness knows how violins do it....

So....a discussion that started with extreme banjo prejudice ends up with some fairly advanced music theory !!

Who'd have thunk it?