Just like with pregnancy with all other animals there isn't a set amount that they should be gaining while pregnant, regardless of the number of pups. Different sows will gain different amounts even if they are pregnant with the same number of pups, simply because some will carry more fluid than others and some pups will be larger than others.
From my experience the birth weights of pups varied on average between 65 grams to 130 grams when first born. The bigger pups tended to be from smaller litters.
The only way to know for certain how many pups she may or may not be carrying would be get an x-ray after she is definitely more than 6 weeks along as by that point the pups bones will have calcified so they will show up on the x-ray. Alternatively you can just wait until she births and see how many pups she has.
Knowing how many pups she may be expecting doesn't really change anything unless she is a sow older than 1 year who hasn't birthed before.