What does "realize SoP function using gates with arbitrary number of inputs" mean?
It means your AND and OR gates are allowed to have as many inputs as you need. For example, if you needed a product term with 8 inputs, that would be allowed. 

If you had been asked to implement the circuit using standard 7400-series logic, for example, an 8-input gate might not be available, and you would have to make an equivalent circuit using several gates with fewer inputs per gate.