When I saw this notice at the front of our local bus I assumed they had made up the word 'standee' but it seems it was simply one which had escaped my notice before.
A standee is somebody who is forced to stand, for example on a crowded bus;
(I still can't make sense of the notice though - I think it was meant to read 1 wheelchair or five standees.)
A standee is also a life-size shape, for instance of a celebrity, cut out of cardboard, often used for advertising and promotional purposes.
"He had his picture taken with a standee of the president"