How to choose number of neurons and number of layers?

Course Q&A Machine Learning Specialization Advanced Learning Algorithms
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why does Andrew choose 25 neurons in first layer and 15 in the second layer?
I think 255 pixel need 255 neurons in the first layer
how to choose number of neurons and number of layers??

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Yes, the input requires one unit per feature.

Selecting the number of units per hidden layer is a matter of experimentation and compromise.

In general you want enough units to get sufficiently good results, but you want as few units (and layers) as possible to minimize the computational requirements during training.

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In addition to @TMosh‘s reply: There are use cases where hyperparameter optimization frameworks like KerasTuner might be worth a look, see also:

Hope that helps, @MOHAMED_AZIZ2!

Best regards
Christian