How many Ethiopians did God help Asa kill?

One million

There came out against them Zerah the Ethiopian with an host of a thousand thousand

Asa cried unto the LORD his God, and said, LORD, it is nothing with thee to help, whether with many, or with them that have no power: help us, O LORD our God; for we rest on thee, and in thy name we go against this multitude. O LORD, thou art our God; let no man prevail against thee. ...

So the LORD smote the Ethiopians. 2 Chronicles 14:11-12