It's very rare that there are even 500 runners on the day, even though the limit is 600. I know a few friends who got injured between the point of the entry details being allowed to be changed and I know people will enter and decide that week if they fancy it or not. Unavoidable, I guess
http://www.scottishhillracing.co.uk/...RaceID=RA-0098
Link above shows a handy little graph on number of runners each year (2005 to 2016)