Yeah, every six months actually. I should do the same to mine. Though I heard not all mattresses can be flipped - in which case you just turn them around.
Hmm not sure if it was 20 but then you dont need to keep flipping mattresses ;p more than enough to just switch every so often~ I do wonder what happens if she accidentally rolled a bit while sleeping ;p
If we're talking spring mattresses, then I'd say about 10 feet, at least? Though in one version I read, the Queen called for 'all the spare mattresses in the castle' and not just 20 but meh, who knows? xD