http://www.epanorama.net/documents/audio/spdif.htmlSPDIF is a one-way protocol that carries its own syncronization/clocking information so theoretically you should be able to split the signal to more than one receiver and they will all stay synchronized (+/- a few samples based on the receivers' internal buffers).
HOWEVER, if you're using electrical connections (instead of optical) you will run into problems. An SPDIF signal is electrically isolated using impedence matched transformers in the sender and receiver. When you splice many receivers together receiver impedence will be lower and the signal from the sender will be effectively weakened (worse so than 1/n). If you had only 1-2 extra receivers you would probably be within the tolerances of your equipment, but at 20 you'll probably exceed the drive ability of your source.
If you're all DIY, you can do a little simple circuitry to (electrically) buffer the output signal and drive as many receivers as you want. I built one that was powered off my computer's PS/2 port and convert the electrical output to optical. It drove a red LED that sat behind my receiver and pointed at the optical input port. That circuit can light as many LEDs as you want subject to power supply constraints. If you go that route, I can give you more details on my setup and how to scale it to 20 receivers.
If you go with a different plan, just remember that digital transmission will not introduce any noise (duh), speaker wire will introduce very little, and that line-level transmission will introduce a lot. You will waste power with long speaker wire runs though it probably won't be significant at normal listening levels (<1W/speaker).
Oh, and if anyone even mentions jitter, slap them in the face. Jitter in modern equipment is a total myth and the cheapest nylon optic cable is (for your purposes) just as good as the highest grade glass telecom cable.
