SELECT LENGTH('amritaseema') - LENGTH(REPLACE('amritaseema', '&Alphabet','')) occurrence
FROM   dual

SQL> def val=eye
SQL> with
  2    letters as (
  3      select chr(ascii('a')+level-1) letter from dual connect by level <= 26
  4    )
  5  select letter||' -> '||
  6         ( length('&val')
  7           - length(translate('&val','#'||letter, '#')) ) val
  8  from letters
  9  where instr('&val',letter) > 0
 10  /
VAL
---------------------------------------------
e -> 2
y -> 1

2 rows selected.

SQL> def val=eye
SQL> declare
  2    type tab_type is table of integer index by varchar2(1);
  3    letters tab_type;
  4    str     constant varchar2(200) := '&val';
  5    c       varchar2(1);
  6  begin
  7    for i in 1..length(str) loop
  8      c := substr(str,i,1);
  9      if letters.exists(c) then
 10         letters(c) := letters(c) + 1;
 11      else
 12         letters(c) := 1;
 13      end if;
 14    end loop;
 15    c := letters.first;
 16    while c is not null loop
 17      dbms_output.put_line(c||' -> '||letters(c));
 18      c := letters.next(c);
 19    end loop;
 20  end;
 21  /
e -> 2
y -> 1

PL/SQL procedure successfully completed.