The OP is probably looking for a average, if she needs to have 2 doz a day regularly she will need to old back a couple days to balance out. But 40 hens is a good number for layers and should take care of her needs, she can always feed back the extras to the layers.
I was going to say that it depends on the breed... I have BO mixes, and they are pretty regular with one a day, so if I had 24 of the same hens, I'd be pretty on target!! lol I'm hoping with an eventual count of laying hens at 18-20 that I'll average a dozen a day. Taking that number, I'd think you'd need at least 30 to expect to get 2 doz/day.