Ответ:

arr = [0, 0, 1, 5, 3, 9, 9, 9, 11] # Initial arrange

arr.append(None)

cnt = 1  # Counter

s = []  # List of amount of consecutive num.

for i in range(len(arr) - 1):

   if arr[i] == arr[i + 1]:

       cnt += 1

   else:

       s.append(cnt)

       cnt = 1

print(max(s))