I don't know if anyone is a SKlearn expert. The documentation is kind of on the lite side. I am working with MLPRegressor. The documentation says:
hidden_layer_sizes: tuple, length = n_layers - 2, default=(100,)
The ith element represents the number of neurons in the ith hidden layer.
From what I have deduces, in the hidden layer size you put the number of neurons for that layer.
So, MLPRegressor(hidden_layer_sizes=(3)) is 1 hidden layer that has 3 neurons. MLPRegressor(hidden_layer_sizes=(3, 4)) has 2 hidden layers, the first with 3 and the second with 4 neurons. MLPRegressor(hidden_layer_sizes=(3,4,5)) has 3 hidden layers with 3, 4 adn 5 neurons.
Is that correct?