The usual 7-octave concert grand has 88 keys, but some Bosendorfer pianos have more. One of my old teachers has one in his house and there are four extra keys at the bottom. The extra keys have reverse colourings, so the white keys are black, vice versa.
A cool story about Vladimir Horowitz about the number of keys on a piano. So, some pianist was playing a concert, which Horowitz was attending. Nameless pianist I can't remember played some Chopin sonata around the end that Horowitz was pretty impressed by. He didn't want to come all the way backstage to congratulate him because that would steal the show, I mean, Horowitz still holds the record for the highest paid classical musician ever. So what he did instead was write a note and have it sent up. A couple of days later the pianist wrote back and said, why thank you very much, but I can never create the sort of excitement you do when you're at the 88. Horowitz is confused. 88? What's that? Is it some club? Bystanding Sonia (his daughter) says, I think that's the number of keys on the keyboard, isn't it Papa? And Horowitz said, well, I wouldn't know, I've never sat down to count them all!
*ba dam chhhhh....*
You can get the full telling on the DVD The Last Romantic, a home-video documentary of Horowitz playing some stuff, talking about music, and telling some stories.