Rule of thumb is around 10. It could be more or less depending on your current weight and how you carry it. For me, it was more like 15-17 per size unfortunately.
I think it depends on where you are with in that size also. For instance if you wear a size 12 but it is a tight size 12 then it will probably take you more than the 10lbs often stated to drop a pant size.
Remember that it is not all about weight. I have been working out steady for the past 3 weeks. The first week, I lost 3 pounds, the second week, nothing and last week I gained 2 pounds. My pants are starting to get a little baggy in front and that is not because of pounds lost but because I have been exercising, my body is changing. I am also hoping to lose 10 pounds and I think it may be slower than I thought but I fell better and my pants aren't fitting so tight around my waist. It feels terrific.
I have to agree that it is definitely not all about weight. It's also about how toned you are. Example: I lost 25 lbs. a few years ago and went down from a size 12 to a size "loose 10 or tight 8". I lost about 5 more pounds and WAHLA! I was a comfortable 6. That was mostly because I lost a flabby 25 pounds and then I was getting toned so 5 pounds was more about inches than pounds (if I am making sense).
