As much as you can.

Look at house prices in your area. The closer you can get to putting 20% down, the better off you’ll be—the less trouble you’ll have getting a mortgage and the fewer other annoyances you’ll have to deal with.

Save every cent that you don’t need for absolute necessities, such as food or utilities. Do not eat out, do not buy any clothes, furniture, or other household items or have any unnecessary medical/dental work.

Buying a house always results in unexpected expenses, so save way beyond the normal 20% downpayment.