multiple multiplexer (4051) - how to process the inputs? I have my Arduiono which has only a limited amount of Inputs/Outputs. But in my project I will have to attach a ton of potentiometers/buttons/LEDs, so I will have to use the IC4051-Multiplexer/Demultiplexer.

I have understood how to switch the Inputs/Outputs via the 3-Bits. But I don't get it, how to connect multiple ID4051s and process the signals(?)

Can someone explain that to me?
I have no experience of this chip but I would guess that you would need to connect the child level multiplexer to the output of the parent and use the outputs to drive the address lines.

You could look at an MCP23008 or MCP23016, these are 8 and 16 bit GPIO expanders that you control via I2C.  You can have 8 of the MCP230008 (64 pins) before you need to look at I2C bus expanders (which you can get).  I think its the same for the 16 bit version, but you would need to double check.