I usually have between 4-5 dogs here all of the time and they are all different.My personal dog is 9 years old,and while she has become a lot more active and certainly enjoys most of the dogs here,I always make sure that she gets her alone time with either my husband or myself. I think if you are going to have multiple dogs,you can't really have a 'favorite' because it's not fair to the other dogs. You will let your favorite dog get away with more than the others. As far as the foster dog getting in your lap and demanding attention, that to me is a training issue, because we never let any of the dogs demand attention from us. If you honestly think you couldn't love a second dog as much as you do your current dog,then maybe you should just have one dog.
I have thought of getting a second personal dog,but with as many fosters as we have come through here,it's not like there will be a shortage of dogs.