According to different research papers, 0,9-1,0 bn. hectares of forests would be needed globally to limit global warming to 1,5-2,0°C. This would presume a strong and sustainable reduction of CO² emissions, to reach carbon net-neutrality by 2050. 

IPCC: The IPCC has calculated, that a growth of 1bn ha of forests (size of USA) would be needed to limit global warming to 1,5°C by 2050. This could result in the storage of an estimated 205 gigatons of carbon, which equals about 2/3 of the carbon that humans have added to the atmosphere since the industrial revolution. This would mean, that the average hectare of restored forest could store 205 tonnes of CO² p. ha. 

Source: https://www.ipcc.ch/sr15/

This has also been validated through different research papers, which found out that an area of 0,9 to 1,0 bn hectares on earth is available for reforestation. More than half the restoration potential lies in six countries, which are namely Russia, the US, Canada, Australia, Brazil and China.