https://www.drmem.com/uk/memory-ram-...s-a515-44-r93g
https://www.crucial.com/compatible-u...e-a515-44-r93g
Seems to say there's 2 available memory slots and it's 2666 or that 3200 would work as well. I would guess a matched pair would be faster than a mismatched but for this level of laptop I doubt you'd notice a difference, though 16GB would be better than 12GB, but I guess you have to decide if it's worth the extra $40 for an additional RAM card.
Opening it up on those plastic clamshell kinds are annoying but taking it easy and watching a youtube video of it before should make it doable without damaging it.