The question of when to add more signs is case specific, but there are many consideration you should make.
You should add signs when your baby isn’t signing yet after significant attempts and when you’re baby is signing proficiently. This might seem counterintuitive, but it’s not. In the fist case, you want to add some signs hoping that you might add a sign or two that your baby is motivated enough to sign back. In the second case, you want to add signs in order to keep your baby interested and growing.
Usually you’ll have a few signs on the go, and a few signs that your baby is using regularly. This is a pretty basic pattern that most parents follow. If you aren’t working on any new signs, then your baby is not reaching their full signing potential. This isn’t a big worry for most parents, but it does mean that your baby can use some extra stimulation, so whether you add a sign or not, is completely up to you. Naturally, my recommendation is to continue to stay ahead of your baby by at least a few signs.
If you’re trying hard to sign, but your baby is not signing back then consider dumping a few signs and replacing them with other things you think your baby might find interesting. You can probably tell what excites them based on your baby’s expression when it’s present, or if your baby points to something out of curiousity. Keep in mind that while you might find certain things benign, your baby might find them totally amusing. Try ceiling FANS and LIGHTS!
Give that a go and see if you can’t boost your baby’s signing vocabulary.