The Extent of the Cultivable Land by Professor C. B. Fawcett estimates that roughly half of the land is cultivable. For Earth, that means 10.8 billion acres.

World Agriculture: Towards 2015/2030 - An FAO Perspective, estimates that roughly 10.3 billion acres of the Earth’s land is ‘some degree suitable for crop production’ (first paragraph of section 4.3).

Global Agro-ecological Assessment for Agriculture in the 21st Century from IIASA, estimates that roughly 10 billion acres of the Earth’s land is cultivable (see the section ‘Land with cultivation potential’).