There’s no lower or upper limit. There are about 15 signs that are desperately needed to all parents to make life easier for you and your baby, so I recommend those first. Aside from learning signs like MILK, MORE, EAT and so forth, you might want to teach signs for the things that capture your baby’s interest. You’ll figure out what those are easily enough! By the way, some parents get hooked on signing and teach 50-100 signs, some even more, however, other parents are happy to teach only a handful, and this is right for them.
My son was up to about 80 signs by the time he began learning words and these started just as slow as did signs. You get one or two works here and there, this grows regularly but slowly until words are added exponentially. There will be a time when your baby adds words so fast you have problems keeping up and this will be just around the time when your baby begins to become more verbal. Do keep in mind though that there will be a fairly long transition from signs to speaking so signs will still play an important role in your baby’s life. During this time, you’ll probably want to add in signs that are difficult to pronounce.