Exercise 6 : Question about backward_propagation:

I took a look in the test dataset. Shape of X is (2,3) so two features and three examples.

Shape of W1 is (4,2). I take this to mean that we have four neurons. However, for the life of me I cant see where have we defined that the number of neurons is 4 Or is it that the number of neurons is the number of rows in W1?

Asking since in exercise 2 we explicitely define layer_size n_h = 4