The impedance of your subwoofer has nothing to do with the number of channels your amplifier needs to have. The impedance is a measure of the opposition to the flow of alternating current. It is sort of like "resistance". All it means is that you need an amplifier that will output the power you need into that impedance.
If you have a 2ohm subwoofer, you want an amplifier that is 1) stable into an impedance of 2ohms, and 2) outputs the power you need into a 2ohm load. Likewise, if you have a 4ohm subwoofer, you want an amplifier that is 1) stable into an impedance of 4ohms, and 2) outputs the power you need into a 4ohm load. Or, ifyou have a final 1ohm ohm (say, a dual 2ohm voice coil subwoofer, with the coils wired in parallel), you would want an amplifier that is 1) stable into an impedance of 1ohm, and 2) outputs the power you need into a 1ohm load.
This amplifier could be a mono amp, a 2-channel amp, a 4-channel amp (if you were going to likewise run your interior speakers off of 2-channels, and bridge the rear channels to the subwoofer), etc.
For subwoofers, a mono amplifier will due since you can't really hear subbass in stereo. So, a 1-channel (mono) amplifier is perfectly acceptable for any subwoofer application. You could likewise look for a 2-channel amplifier and bridge the amplifier to your one subwoofer. However, if your subwoofer is a low impedance (2ohms or lower), then you may have a hard time finding a 2-channel amplifier stable into that impedance since not many are stable below 4ohms bridged. Likewise, you could use a 4-channel amplifier and do as I described above....run your two front speakers off of the front two channels of the amplifier, and bridge the rear two channels to the subwoofer. But, again, if you have a lower impedance subwoofer you options in 4-channel amplifiers will be severely limited since not many are stable into 2ohms bridged, and even fewer into less than 2ohms bridged.