How many would it take to remove ~40-50 giga-tons (40x10^12 kg) of CO2 per year? Two trillion (2,000,000,000,000) trees per year. The total number of trees on Earth today is estimated to be about three trillion.
So we would need to plant ~2 trillion per year to just keep up with the amount of CO2 being produced today. Do that many saplings get produced every year? How much land would it take to create that many saplings? How much time and labor? And what steps should be taken so that none of them end up rotting once they are dead (and release the CO2)?
How many more would it take to use the biomass to create something that can offset the existing fuel usage that keeps billions of people alive?
Hence the reason to pursue more efficient forms of carbon capture than just "plant a tree"