[baq]

Nonono. We absolutely should recommend having at least 2. See also: car keys, house keys, any other physical lock you can get comes with at least 2 keys.

The default product sold should be a two key bundle.

[damnyou]

With your house keys you can walk into any hardware store and get as many duplicates as you like. With a U2F key that is prohibited.

U2F desperately needs a layer of indirection that doesn't currently exist.