Yes, you can use this to enter a number of servings. If you have an entry for one serving and you want an entry for two, you'd put "If this item is _1_ units, give me values for _2_ units." (In practice, I often enter the one-serving item twice, unless I'll regularly be having two.)
Similarly if you have values for 100 grams, and you're going to consume 50, you could do "If this item is _100_ units, give me values for _50_ units." (or 10 and 5 to save typing, or even 2 and 1 -- the result would be the same.)
I hope that makes sense!