How many 8-ohm speakers can connect to the output of an amplifier which has outputs of 8 ohms without having quality or reproduction sound problems?
If the output of your stereo is 8 ohms you can only connect an 8 ohm speaker, so that the impedance is maintained. If you connect a 4 ohm speaker, or two of 8 in parallel, the impedance drops to 4 ohms and with that you overload the output circuit of your sound system and you can burn it. On the other hand, if the output of your sound system is 4 ohms, you can connect an 8 ohm speaker (with less power), a 4 ohms or two of 8 in parallel, because then the impedance will remain in 4 ohms. See then that the speaker set that you connect to the output of your sound system should always have a total impedance equals to or greater than the amplifier output so that it is not overloaded.