How many select lines? If you speak in the most general sense, it would depend on how the multiplexer was designed. You could design the select input circuitry in any way you’d like. You could have a billion lines or 1 line. But usually, people code the select lines as a binary number because it’s compact and easy to drive from other digital circuitry. So what’s the fewest number of binary digits required to make 16 unique combinations?? That would be your answer.

The last question needs some more info to answer it. A decoder takes a single input and fans it out to several different outputs. Minimum being 2 per input. If the decoder has 5 inputs, then what you’re describing is actually an array of 5 decoders each with it’s own set of inputs and outputs. Therefore the minimum number of outputs on this device would be 10, but there is not enough information in your question to accurately answer.