How to change listing per page option?
Go to admin
Store > Configuration > Catalog > Storefront
Now there are two things
Products per Page on Grid Allowed Values
Enter your desired no here with comma like
Products per Page on Grid Default Value
Value you enter here must be defined in Allowed Values.
like you can enter from these values only
See how it looks after changing
Clear your cache and it should work smoothly
you can change in admin Store >> Configuration >> Catalog >> Catalog >> Frontend TAB
You can change it via admin panel in section Stores -> Configuration -> Catalog -> Catalog.
You are interested in the fields Products per Page on Grid Allowed Values and Products per Page on List Allowed Values.