my_list = [42, 45, 67, 89, 99, 10, 23, 12, 31, 43, 60, 1, 0]

print("The list is :")
print(my_list)

my_key = 3
print("The value of key is ")
print(my_key)

my_result = []

for index in range(0, len(my_list) // my_key):
   my_result.append(my_list[0: (index + 1) * my_key])

print("The resultant matrix is :")
print(my_result)

The list is :
[42, 45, 67, 89, 99, 10, 23, 12, 31, 43, 60, 1, 0]
The value of key is
3
The resultant matrix is :
[[42, 45, 67], [42, 45, 67, 89, 99, 10], [42, 45, 67, 89, 99, 10, 23, 12, 31], [42, 45, 67, 89, 99, 10, 23, 12, 31, 43, 60, 1]]