- global amount of co2 emissions per year
https://www.statista.com/statistics/276629/global-co2-emissi....
- average co2 sequestered per tree - 25-45 kg depending on species (google for estimates)
https://www.usda.gov/media/blog/2015/03/17/power-one-tree-ve...
37 billion metric tons / 25 kg = ~1.48 trillion
Amount of land available for reforestation as of 2019 is enough for 1.2 trillion native saplings.
If you start with completely empty land, at first you will have increase in biomass, but after some time for new trees to grow you will need to have some other ones to die. The trees might keep exchanging CO2 to O2, but you will also get steady increase in emitted CO2 either through forest fires or microorganisms or animals eating and decomposing and literally breathing the CO2 back to atmosphere.
When things are in balance, forests do not actually remove any CO2 because any CO2 removed is balanced out by new CO2 emitted. Only planting a new forest on a new land binds some new CO2, and that only temporarily until the forest matures.