Yes, I honestly know what you mean. All the fosters are wonderful, but then the extra special ones pull at your heartstrings. I loved Blue, and almost burst into tears when she was chosen at the adoption show. But she became cat # 3 in a couple's home, very doted on and spoiled. At my house she would have been cat # 6, not counting the current 11 fosters. Truly, it was best for her to go to another home.
But Festus and I miss her. Festus mourned when Blue left, and layed on a chair with her head down. I felt terrible!
Then there was Midnight, who I bottle fed. It almost felt like giving up a child when he left. It didn't bother me much to adopt out his siblings, but he was so precious!
But it only takes keeping a few to have too many cats. And to me, more than 3 housecats is too many, based on my ability to keep litterboxes clean, give attention, and provide vet care.
If you have room for one more, go for it. But remember you may end up with a difficult to adopt kitty with special needs...like my Jazz. She just stayed feralish. She would have hated the adoption shows, and been scared to move to a new home. We've had her for a year now, and yesterday she let me pet her more than once!
DD even picked her up a few days ago! (Just to clarify for those who don't know the story of Jazz and Blue...we did adopt Jazz. So whether or not she turns into a total lovebug like her sister, she is ours...and definitely not available for adoption!)