Whomsoever – singular, regarding an unknown individual to be specified later.
The term occurs 20 times in Scripture. 10 times to Israel in the Old Testament, 8 times to Israel in the gospel accounts, once to Israel in Acts, and once to the Body of Christ in the Dispensation of Grace, and none following the Dispensation of Grace.
Mark 15:6 Now at that feast he released unto them one prisoner, whomsoever they desired.
See also Whosoever, Whose Soever