There is no 'right' amount of weight to put on through pregnancy.
I have heard that the 'average' weight gain is from 10-15kg.
Don't forget that our bodies actually need the extra fat stores when pg, so even if you are putting on fat weight as well as baby weight, some of that is still necessary.
It also depends on how much you weighed before you fell pg.
Sometimes its common for women who are overweight before pg to gain less weight during their pg, and for women who are underweight before pg to put on a lot more.

I was about 5-10kg underweight before I fell pg, and ended up gaining 40kg in my pg (but that was due to a lot of other factors as well).

As long as you are eating healthily, doing moderate levels of activity or excercise, and bubs is healthy... then there is no need to worry about how much you gain during pg.
I have actually heard that there are heaps of OB's/midwifes that never weight their pg women clients at all.

HTH