4!

If we retreat we lose cohesion. That can't be suffered now. We must stand our ground or fail.
Also a retreat will not even buy us time as the Zulu will likely follow us fast and we risk losing those who somehow fail to recieve the order.
"Not one step backwards!!!"

2000 vs 700 is perhaps not good, but we haved ranged weapons and some very good shooters. We should be able to deplete them enough to fail their charge.
And if that isn't enough, well, then nothing would have been enough...