is it better to use 2 or 4 ram slots - Can I use two 4GB RAM sticks with 1 8GB ram stick on 2 dimm slots ... samurai 888 katsumi slot machine

One cpu pin is bent and I want to know which cpu pins affect ram slots beacuse pc doesnt boot with 2 ram sticks in slot 2 and 4 only boots when there is one ram on 2nd slot.

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.

So my Laptop is a Lenovo Ideapad 300-15ISK, i got it back 2016, on task manager it says it is got 4 ram slots but when i open the panel on the back i visibly only see 2 ram slots, no they don't stack and would they be deeper on motherboard or under more panels? and second question i am not sure...