Change the range and change 50 to number of cells in range

e.g if range is 8 columns and 15 rows (120 cells in total ) then 50
becomes
120:

=LARGE($A$1:$H$15,INT(RAND()*120+1))

And if you don't want to have to worry about the cell count, use this
variation and just change both ranges each time you make a change in the
number of cells included...

=LARGE($A$1:$H$15,INT(RAND()*COUNTIF($A$1:$H$15,"< """)+1))

Rick