declare
  c clob;
  n int;
  s varchar2(32767);
begin
  dbms_output.put_line(systimestamp);
  c := 'select count(*) from (';
  for i in 1 .. 40 loop
    s := '';
    for j in 1 .. 500 loop
      s := s || case when i*j > 1 then ' union all' end || ' select '||i||' as c1, '||i*2||' as c2, '||j||' as c3 from dual';
    end loop;
    c := c || s;
  end loop;
  c := c || ')';
  dbms_output.put_line(systimestamp||' l='||length(c));
  execute immediate c into n;
  dbms_output.put_line(systimestamp||' cnt='||n);
end;
/

15-APR-21 11.55.34.705903000 AM +03:00
15-APR-21 11.55.34.739614000 AM +03:00 l=1129193
15-APR-21 12.02.24.489920000 PM +03:00 cnt=20000

PL/SQL procedure successfully completed.