Hey there
I am currently working on a project where I need a bunch of relais.. And these are about 12$ i thought about ac multiplexing.
I saw this: https://forum.arduino.cc/index.php?topic=19524.0 but since this is 9 yrs ago i thought about asking again so I did. is the MAX4508 still the thing to go? Or which multiplexer should I use?
Thanks! 
This is an XY problem. You are asking us to help you with your solution to a problem. How can we say that your solution will work if we know nothing of the problem. Perhaps there is a better solution.
What are the relays driving?
What are the rated voltage and current of the relay coils?
Do the relays have built in drivers?
A schematic would probably help.
Years ago I worked on a project that used the MAX4509 to switch audio sources. cool chip.
Each switch pin of the MAX4508 is limited to 30ma, so as Fungus said.
What is the problem you are trying to solve?