Hornbills:
Silvery Cheeked
Trumpeter
Piping
Northern Ground
Southern Ground
Black Casqued
White Crested
Red Billed
Von Der Deckens
Jacksons
Yellow Billed
African Pied
African Grey
Eastern Yellow Billed
Southern Yellow Billed
Crowned
Using your list as basis:
Silvery-cheeked: 60+. Often kept off exhibit as a program animal, though a few zoos which exhibit them include Omaha, Tampa, and perhaps Pittsburgh (did anyone ever figure out whether they’re actually there?)
Trumpeter: definitely around, but not as common as some species. I’ve seen them at Dallas and Gladys Porter, and perhaps others.
Piping: I know nothing of their population in North America.
Northern and southern ground: breed pretty regularly to my knowledge, and can be seen at quite a few zoos each.
Black casqued: I’m only aware of these being kept at SDZ and perhaps Boise.
White-crested: Bronx, Central Park, and SDZ keep the nominate subspecies. They are around privately.
Red-billed: not sure on the status. Any pure ones hanging around?
Von der Decken’s: again, not too sure, but I’ve seen them at Maryland, Smithsonian, and Kansas City.
Jackson’s: only a pair at Dallas to my knowledge.
African pied/grey: I’ve never seen them at or heard of any NA zoo with them, but both are around privately.
Eastern yellow-billed: seems to be the most common of the Small African hornbills in public collections, in my experience.
Southern yellow-billed: only Cameron Park Zoo seems to keep it in the US, unsure otherwise.
Crowned: seems to be one of the rarer species. They are kept at Houston and San Antonio.
Yellow-casqued: a pair is kept at Memphis.
White-crowned: a pair is kept at St. Augustine.