How many lambs you can have per acre depends on a number of things, such as what area of the country in which you live, and what kinds of grasses you have. That determines what kind of grass growth we're talking about and when it grows. Also, what kind of imput are you wanting to put in? Would you be interested in intense rotational grazing to maximize the number, or are you wanting to put them all in a big field from day one till finish? Or perhaps something in the middle...
In addition to considering the amount of forage you'll have, you also need to think about how you will keep parasite levels down to keep growth rates up. This is more difficult when you only have one lamb field, since you can't put the sheep on fresher pasture after you worm.
I have a book to recommend...about making the most of grass with lambs...but I'm tired and can't remember what its called! I have it upstairs, so I'll find it and give you the title and author tomorrow (hopefully I'll remember!).