How to use neural networks with large and variable number of inputs?
There exist many possibilities for populating empty gaps on data.


Most repeated value: Fill the gaps with the most common value.
Create a distribution: Make the histogram and drop values according to that distribution.
Create a new label: Since you do not have information, do not assume any value and create another label/category to indicate that value is empty.
Create a classifier: Make a relation among the variable with empty gaps and the rest of the data and create a simple classifier. With this, populate the rest of the data.


There exist many others, but these are the most common strategies. My suggestion is not to populate and to keep unknown what is unknown.