input: Write a python function to count the occurrences of a given number in a list.
  assert count_occurrences([1, 2, 2, 3, 4], 2) == 2
  assert count_occurrences([1, 1, 1, 1, 1], 1) == 5
  assert count_occurrences([5, 6, 7, 8, 9], 3) == 0
output: def count_occurrences(lst, num):
      return lst.count(num)