His 16 championships include 2 WWF title reigns, back in the early 90s. Actually, he won the NWA title, the WWF title, and the WCW title at various times to add up to his total of 16 championships.
He was also given the WCW title once, by Kevin Nash, and for about 45 seconds they were calling him a 17 time world champion. Then he had a "heart attack" in the ring, and that 17th title reign was never heard of again. Just a footnote in the pages of history...