1. use a substitution variable '&' to take the input
   from user.
2. use the EXIT statement to exit.
SQL> ed
Wrote file afiedt.buf

  1  begin
  2  for mag in 1..&max_value loop
  3  dbms_output.put_line(mag);
  4  exit when mag=10;
  5  end loop;
  6* end;
SQL> /
Enter value for max_value: 30
old   2: for mag in 1..&max_value loop
new   2: for mag in 1..30 loop
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PL/SQL procedure successfully completed.