Notice the different coloration of the grass. That is usually a sign they were taken at different times. Although it would be quite a revelation if it could be shown that all those pictures were taken at the same time, since the far away picture with hangers in the background clearly shows yellow patches that are not present on the other pictures. If they were taken at the same time, then that 'proves' there are at least two flying prototypes (and a third ground test airframe, as is standard with Chinese fighter development).
