is it better to use 2 or 4 ram slots - I have 16GB RAM but it's only showing 8GB. 888slots bonus ohne einzahlung

So I have 16GB RAM. After upgrading to Windows 10 however it says I only have 8GB. When I look in task manager it seems to know I have 16GB but for some reason only using 8Gb of it and for slots used it says 2 of 4 when I'm using all 4 RAM slots. I'm not techy at all but if somebody could explain to me why this is it would be appreciated. If it is something like just a broken RAM stick then that's easy to fix but I don't want to go out and buy a bunch of new RAM if it's a different issue.

Hi, im planning to buy 8GB RAM memory for my ASUS X551CA laptop. How to install the 8GB RAM Memory in X551CA Motherboard ASUS Laptop? and what is the limit of Memory Slots of ASUS X551CA Motherboard? is it 4 or 8 GB only? Thanks - Asus X551CA

My friend told me that 2 sticks of 4 ram are better than 1 stick of 8 ram is this true? if it is how is it better?