Just store the value in an ini file.

Code:
on *:text:!pizza:#:{
  var %file = commands.ini, %cmd = $1
  var %sum = $calc($readini(%file,n,$nick,%cmd) + 1)

  writeini %file $nick %cmd %sum
  msg # $nick has now eaten %sum pizza $iif(%sum > 1,slices.,slice.)
}