How many you will need depends on how your game is set-up
Primarily perspective and camera control, if you tweak these a little you might be able to makes 8 direction look good.
If you have 8 directions that means your view angle can shift up to 45 deg without your sprite changing, this will stand out as even a small shift in any non-pre-rendered terrain object will be very noticeable in comparison
One game I remember was harvest moon 64, I forget how many views it had but I think it was more than 8 and just running around looked fine but as soon as your rotated the camera the pre-rendered objects stood out allot
You should try first and see how it looks as soon as you get the graphics engine going, It might be ok. If not you could try not letting the player manually rotate the camera or making it so it doesn’t do a perfect circle around the player(the later could be anoying but you might be able to make 4 directions work)

I was checking around through some of my old video games and I managed to find one that does 4-directional movement using 2D sprites on a 3D terrain..

Well this game did it and it worked really well, allowing full camera rotation with the character face-direction changing every 90 degrees..

Since the characters were pixelled (the exact same method I will be using) it didn't detract too much from the quality of the game and it didn't really look naff or out of place at all..

I'll see if I can find any screenshots of the game to post here..

NEways from this observation (and also considering the fact that Ragnarok Online uses 8 directions and its more than enough to keep the characters from looking flat and without depth) i'm thinking that 8 directions should be pretty sufficient..

Altough you mentioned a game that uses more than 8?
Could you point me in the direction of some links or something because I'd like to see how it looks since i've never seen a game that does this before..