Hello! I don't know if this is the right section to post this, if not I'm very sorry about. Everyway I try: I'm using Arduino UNO from some years, and now I'm realising a system using two sensor. The system would be expandible, and so i decided -to learn- to use a multiplexer, and especially a 74hc595b1, leaving some pins free. Can you advice me a simple example to learn to use it? And before, Is it possible? Thankyou! 
74hc595 is not really a multiplexer. It may not be useful for your project, depending on the type of sensor you plan to use. Tell us more and we can tell you more.
hc595 is serial in parallel out shift register. This basicly means it works as OUTPUT pin expander - it cannot work as input. You set all 8 pins as you wish and they stay this way until you set them to other state. Multiplexer connects one Arduino pin to one of 4/8/16 (depends on multiplexer) pins as if you connected them with wire. The rest of pins are connected to nothing. So you control only one of the pins and rest are floating - not connected to anything.