"people say my mother prefers 2 or " - hmmmmm..... intesetng.
It depends on your motherboard. If its dual channel, then you n3eed to have a dual channel kit. If its triple channel then a triple channel kit.
A ram kit is a collection of ram sticks that were tested and found to work together.
But in your case if you have a triple channel kit - 3 x 4gb, you can use 2 sticks to make a 2 x 4gb kit because they have been tested and found that they work together. But if you add more ram to them then you will no longer have a tested kit. And so there's no guarantee that they will work together - even if they are the exact same make and model and have the same timings, voltages and clock speeds.