declare 
    cSQL VARCHAR2(2000);
    qryCtx dbms_xmlgen.ctxHandle;
    result clob;
    k pls_integer;
    r pls_integer := 20; -- по 20 строк
begin 
    select count(*) into k from chek;
    for i in 1..k loop
     if(mod(i,r)=1) THEN 
      cSQL := 'select * from (select rownum r,t.* from chek t) where r between '||to_char(i)||' and '||to_char(i+r-1);
      qryCtx := DBMS_XMLGEN.newContext(cSQL);
      result := DBMS_XMLGen.getXML(cSQL);
      DBMS_XMLGen.CloseContext(qryCtx);
      DBMS_OUTPUT.PUT_LINE(result);
     end if;
    end loop;
end;