Leucistic birds are pretty rare, but a leucistic hummingbird? Wow! According to the Cornell Lab of Ornithology:
From 2000-2006, Project FeederWatch participants reported fewer than 1000 leucistic birds. Given that participants report about 5.5 million birds each season, the percentage of leucistic birds being reported is very small.
If your wondering what the difference between leucism and albinism is, this is how the Cornell Lab of Ornithology defines them:
Albinism is a genetic mutation that prevents the production of melanin in the body. Leucism is a genetic mutation that prevents melanin from being deposited normally on feathers.