According to Wikipedia--Individual white buffalo--a couple dozen white buffalo have been born in the last century. For example:
A male white buffalo named Spirit of Peace was born on April 17, 2005, on the Blatz Bison Ranch in Fort St. John, British Columbia. Spirit of Peace died on June 1 of the same year, probably as a result of his premature birth.
A female White Buffalo calf was born in Shelbyville, Kentucky, on June 3, 2005, at Buffalo Crossing, a buffalo ranch and tourist facility located. She was named Cante Pejute (Medicine Heart in the Lakota language) in a traditional ceremony led by Steve McCullough, a Lakota/Shawnee from Indiana.
A male named Blizzard was born in March 2006 on the farm of an anonymous rancher, who arranged to have the calf transported to Assiniboine Park Zoo in Winnipeg, Manitoba in recognition of his spiritual significance to aboriginal people.
A number of questions come to mind. What if a white buffalo calf dies young--does it still count? What if it's a light beige rather than pure white? What if its fur turns brown when it grows up? What if it's an albino (missing pigmentation) rather than colored white "naturally"? What if it's not a pure-bred buffalo--if it has cattle genes?
For more on the subject, see White Buffalo Calf Born.
Below: "A white buffalo at the Lee G. Simmons Conservation Park and Safari in Ashland, Nebraska. This animal is not a true white buffalo, being 1/16 Charolais cattle. It is expected that its coat will darken as it matures.