A Voice within a synthesizer is regarded as a single note that can be played. You will find that monophonic synthesizers are only able to play one voice/note at a time, whereas polyphonic synthesizers are able to play multiple voices/notes at any one time.
Analog synthesizers are usually monophonic (capable of playing one note at a time) because of the circuitry required for each path signal. This type of synth would require a separate signal path made up of oscillators, filters, modulators for every note played.
Therefore, you can see that trying to wire up an analog synthesizer with polyphonic qualities would get very complicated and take up a lot of space, meaning the resulting synthesizer would be very bulky and not very portable at all.
Digital polyphonic synthesizers can handle the multi-voice capabilities because of the way they are processed via the internal CPU. Space is less of an issue and so multiple voices can be made available and played at the same time. The common number of voices range from 4-24.
It is useful to know that the number of voices available is not always exact and will depend on the type of sounds you are playing. For example, if you are using a synthesizer with 64-voice polyphony and you have a complex, layered sound that you created from four simpler sounds.
This layered sound is actually using four notes at at any one time, so the original 64-voice would be reduced to just 16-voice. It’s very rare that you would use all voices anyway but it’s useful to understand how voices work in polyphonic settings.