The best I've managed so far is saving 16 18 of the cows.  I also played it evil a few times and managed to have the tornado suck up all 20 of them.

Just a few notes:

• You might occasionally see the tornado seemingly get stuck in one spot, usually along the fence.  It's not really stuck.  Sometimes it just decides to wait there for a while.  Tornadoes are unpredictable like that.
• Herding isn't an exact science, and some cows are more stubborn or panicked than others.  But in general, if you position yourself on one side of the cow, it will try to go in the other direction.  Unless it gets distracted and decides to follow another member of the herd instead, of course.
• The cows are dumb.  They don't know what a tornado is.  Some of them might go check it out.  Most of them don't seem to know it's there.  None of them run from it.
• There's a teleporting cow bug.  It doesn't happen often.  If I wasn't over time I would have fixed it.  It's actually a very rough error correction routine - if a cow gets stuck in one spot for too long, instead of risking you ending up with a cow that you can't herd because it's stuck, it just moves the cow to a clear spot to keep the game moving.