How can I increase the max number of pieces for the mutations? I try to go further than 4 but it doesn't change when I click on it.
You can use the directional buttons (left/right/a/d/whatever you have your keys set to).
This is because of a limitation where I wanted to keep right click as a way to exit the menu, so clicking would only have been one direction (positive) and that's not ideal. Also can't be up and down directions because those change what item in the menu you have selected.