Currently, Stackexchange claims 53 Q&A sites. I count 55, including MSO. I'll grant that MSO ought not to be counted, given that we don't count the other metas. But that still leaves me at 54, not 53.
Count for yourself: http://stackexchange.com/sites
(I counted the occurrences of the word 'ago' on the page, I didn't do it manually.)