It's different for everyone, depending on lots of different factors. So, impossible to say.
If you are able to eat healthily (you don't really need to eat for two) throughout, and keep going with whatever exercise you would normally do (take advice from your midwife/GP if you're worried about a specific exercise) then you'll only put on the weigh you need.
When the baby arrives many women find that breastfeeding can help the weight drop off. Not always though. I found going out for walks with baby in the pram (or in a sling) was helpful too. They would go to sleep and I got the benefit of the exercise. There are exercise classes you can go to where you take your baby along whilst you exercise.
It all depends on what you do post birth to tackle any baby weight you want to lose. I am hoping to get out walking most days with the buggy or sling, I don't want to cut my calories back too much as I will be breastfeeding but, I will certainly be eating healthier.
It also depends on how much you put on, some people lose a stone or note just giving birth. I've always been petite and have put on nearly three stone.