The multiplier will increase the size of the screen shot.
For instance, with one screenshot that I did the default 1 would give me a screen shot size of 982x923 and adjusting that up to 4 would give me 3928x3692.
The higher the resolution multiplier is set the longer the screenshot will take to render.
There isn’t really an ideal number for this. If you see that you’re not getting the resolution you want in the default, I would increase incrementally by 1 until you see the result you like.