The manual states that you should not go below a 4 ohm load. This means, if you have two speakers, you can wire them in series of parallel. Wiring in parallel will give louder messages at a given set volume.

If you have more than 2 speakers, then you must wire in a parallel/series fashion so that the net impedance is greater then 4 ohms but as close to 8 ohms as possible. For example, for 4 speakers, wire two sets of two in series then wire the two sets in parallel. This will net 8 ohms.

Your question was how many can you wire. The answer is that you can wire as many as you want, justy don't go below 4 ohms. However, the more you wire, the less volume you will get from each speaker. So you will reach a point where you can't hear anything. So for this reason there is a limit. I don't really know what it is, as I haven't tried it out, but I'd guess you could put 4 with no problem, and 8 should also be no problem, at 16 I'd guess the volume would be getting a little low.