"It is in fact quite easy to visualize that there are 3 voices and this is important when playing the piece."
I could tell the rests (not "stop signs" btw, sorry) were because there was more than 1 voice, but not that there were 3 specifically. How can you tell the exact amount, and why does it matter when playing the piano without any accompaniment?
Perhaps you could recommend me teaching material (preferably online for free) where I can l learn more about voicing for the piano and how it affects performance? Thanks!
Well that is what the rests says; in my edition the first beat has 2 rests for the 2 silent voices for example but of course you should look at the other measures of the piece and it clearly alternates bars with 2 voices and bars with 3 voices. Most baroque music is composed by using counterpoint, in particular all polyphonic music. So you will have 2, 3, 4 (and occasionally more) voices. It is important to follow the voices because unlike homophonic music, any voice can take the lead, and in order to make the polyphony clear the player must carefully balance the weight and importance of each voice. But in classic or romantic compositions, sometimes the inner voices take also precedence and needs to be highlighted.
I am not too much of an online guy, but I am sure others in this forum can direct you to some relevant materials.