If 8 or 16 is enough depend on what sounds you want to play... but it never hurts to have 16 instead of 8.
If you have 8 voices, then if you hold down 9 keys in a chord, the 9th note would not have a free voice to play the sound, as it only has 8 voices... instead that 9th note will steal one of the other 8 held keys that will then stop playing as it is used for that 9th key... that is what is called "note stealing".
So how often will you run into this "note stealing"? ... that depends... it's rare to hold down 9 or more keys... you only have 10 fingers... so if the sound you are playing was a sound that start abruptly, and ended abruptly when you released a key (like en Organ sound for example), then 8 voices would be more than enough.
But what if it was a piano sound that fades out slowly after you have released a key? .. in that case the voices would still be sounding even if you released the keys... so if you then press another chord while the previous still was fading out, then those voices would be hijacked by the new chord, abruptly stopping the old chord as the new one uses the same voices.
This is where 16 voices is better than 8 voices because you can hit a new chord, and use new voices instead of those already assigned to the old chord that is ringing out... so; the more voices, the less chance of voice stealing.
On top of this, when you use "stacked mode" with say; a REV2, it actually use TWO voices per note you press down, one voice for layer A, and another for layer B... so if you have a REV2 16 voice synth, and use it in stacked mode, then you in essence only have 8 voices in that case, since each key uses up two voices.
Personaly, I'd say that the chances that you will run into note stealing at some point is high... even with 16 voices it can happen with sounds that have very long decaying times after the keys have been released. So if you plan on only playing Organ, Leads and basses, plus other sounds with a short release time, then 8 voices might be enough... but if you plan on playing huge pads and other sounds with long release times, you'll probably run into note stealing problems.