This is an extremely difficult question to answer, but as PSM1 points out, it is not so much about the quantity of speakers, as it is about the resulting load impedance of adding those speakers to the amp.
Speakers in series add -
Rt = R1 + R2 + R3 + ...
Example, 6 ohms, 8 ohm, and 8 ohms -
Rt = 6 + 8 + 8 = 22 ohms
Speakers in parallel divide -
In this case, we take the impedance of one speaker, and divide it by the numbe of speakers -
Rt = R1/n
But this assumes speakers of equal value.
Example, three 8 ohm speakers -
Rt = 8/3 = 2.67 ohms
For TWO speakers of unequal value, we use this formula -
Rt = (R1 x R2) / (R1 + R2)
Example, 6 ohms and 8 ohms -
Rt = (6 x 8) / (6 + 8) = (48) / (14) = 3.43 ohms
MOST, amps can tolerate a load per amp channel of 4 ohms to 16 ohms. If ti drops below 4 ohms, the demand for electrical current gets to high, and the amp tends to overheat. If the impedance goes above 16 ohms, the amp tends to become unstable, and is prone to self-oscillation, which is similar to feedback.
Now some cheap amps have a problem with loads as low as 4 ohms. Some better amps can tolerate total per channel loads slightly below 4 ohms. The best consumer amps, as well as commercial PA amps, can easily handle very low loads, but you are not likely to encounter amps like this.
Though there is one exception, NAD amps. Even the very low cost NAD C315 has its dynamic power rated as low as 2 ohms, though I would certainly not recommend trying to run that low. But it indicates that the amp would be very stable running difficult 4 ohms loads, and should even tolerate a 6 ohms combined with an 8 ohms speakers.
So, it is not about the number of speakers, but about the combined impedance load that those speakers present to each amp channel.
In your case, the amps seems to be tolerating a pair of speakers on each channel nicely. Most amps have protection circuits, so if you are overheating the amp, or running it over current, it should shut itself down safely.
Steve/bluewizard