Double-U is probably the correct spelling of the name of the letter w. Some will champion doubleyou (no hyphen, and it actually looks like a real word), but this hasn't really caught on. Some other languages are a bit more sensible, and in German it's called We (pronounced /veː/) and in Dutch it's wee (pronounced /ʋeː/). English went in another direction.
Of course, the overwhelming consensus in common usage is that the way to write out double-U is simply as W, making it one of the least phonetic written words in the English language; none of the sounds used in saying the word are /w/, and it has seven 'ghost sounds' which are said but not written.
In a massively unsatisfying compromise, some go with 'dub', usually when faced with the linguistically nasty www of the World Wide Web or in naming an institution (six different colleges and universities with the initials UW go by the nickname 'U Dub'). Don't do this, please.
Neither of the longer English alternatives have a nice-look plural: double-ues vs. doubleyous. Dubs is just stupid. W's will open you up to the debate as to whether plurals of letters should have apostrophes (they should), so the most common plural is Ws.