The simple answer is just as many as you want! There’s no right or wrong way to teach baby sign language and there are certainly no right amount of signs to teach. Nor are there too many signs. While we talk more about this in the sign course, overwhelming your child is not a huge concern.
So as you begin, start with the main words that have meaning to your child, the things that happen ever day and the things that will make your life easier. I offer some suggestions and techniques in the online course. My son signed 180 words well before he was 2-years-old and that’s not because we forced him to sign. In fact, we just signed whenever our boy showed interested in talking about something. It came to the point where he would look at something, point to it and appear wanting of a sign. This made things really easy for us. After your baby learns enough signs, they can be easily and effortlessly added, which puts the onus on you to continue his learning. That means his instructor (you) will be the deciding factor as to the number of signs your baby will eventually learn.
As for a minimum, I say sign 20 and see how that goes. Keep in mind that the first one is the most difficult, the next probably equally so, if not a bit easier. Once your baby has mastered 10 or so, the next bunch come really easy. Once you’ve got 30 or more, things become second nature. After 100 signs, your baby is a grand master signer and he can add signs as if it was common sense (which it is).
The take-away message is that you should teach just as many signs as you want. While there are many benefits to signing, and these should provide some motivation, signing is about connecting with your baby on a more meaningful level much sooner than you could otherwise. If you aren’t having fun signing, then don’t push it – take a break and if you feel up for it, add some more signs later on.