I don't think PADI requires any dives for Nitrox anymore. A friend just got certified, and I don't think he did any dives, just classwork. I had to do two for SSI about a year ago, and those are my only nitrox dives so far. I haven't done a true dive vacation yet where I'd see the benefits of increased bottom time etc, and it's just not worth twice the price to dive the same dives in my local quarry.