Insert data into nested table plsql

This:

emps := employee_type(sal);

initializes the collection with a single element containing the salary. Every time you do this, you erase the existing collection and replace it with a new one consisting of one element only again.

Instead, initialize the collection once and then extend it element by element:

DECLARE
  TYPE employee_type IS TABLE OF employees.salary%TYPE;
  emps employee_type := employee_type(); -- initializing the array right away
  i NUMBER(5) := 100;
  sal employees.salary%TYPE;
BEGIN
  LOOP
    SELECT salary INTO sal FROM employees WHERE employees.employee_id = i;
    emps.EXTEND(1); -- add one element
    emps(emps.COUNT) := sal; -- fill that element
    i := i + 1;
    EXIT WHEN i > 110;
  END LOOP;

  SYS.DBMS_OUTPUT.PUT_LINE(emps.count);
END;

It is much easier though to bulk collect the values into the array (which also implicitly initialized the array).

DECLARE
  TYPE employee_type IS TABLE OF employees.salary%TYPE;
  emps employee_type;
  i NUMBER(5) := 100;
BEGIN
  SELECT salary BULK COLLECT INTO emps 
  FROM employees 
  WHERE employee_id BETWEEN i AND 110;

  SYS.DBMS_OUTPUT.PUT_LINE(emps.count);
END;