How many neurons for a neural network?

A neural network is a particular model that tries to catch the correlation between the features and the target transforming the dataset according to a layer of neurons. There are several books that have been written around neural networks and it's not in the scope of this article to give you a complete overview of this kind of model. Let me just say that a neural network is made by some layers of neurons. Each neuron gets some inputs, transforms them and returns an output. The output of a neuron can become the input of the neurons of the next layer and so on, building more and more complex architectures.