I kept a backup for 20 years or so, but currently don't have one. I've never had a keyboard go out on the job (or anywhere else for that matter). The Solton X1 flickered and locked up a time or two, but never quit.
The PSR9000 screen locked up once or twice, but it kept playing and cleared up after rebooting. I suppose I usually don't keep them long enough for much to go wrong.
I've accidently blown speaker drivers on p.a. systems, had mic and speakers cables go out, but never a portable keyboard.
I do have music on the Creative Labs Nomad Jukebox that I could use if it does happen, so I guess that's my backup right now.
I keep extra cables, mics, and power cords, and usually even a spare p.a. system in the van.
DonM