Under certain circumstances, you can culture 4 cups of milk with as little as 1/2 teaspoon of grains. However, you will want to adjust the amount of grains to avoid over-culturing and to impart the best flavor.
You don't have to make a full quart of milk kefir at at time. Some people have found that making 1-pint batches better meets their needs.
To learn more about how to slow down your milk kefir or make smaller batches, read our tutorial How to Slow Down Milk Kefir + Make Smaller Batches.
Standard ratio for a one-quart batch:
1-2 teaspoons of grains for up to 4 cups of milk.