We could reasonably plant ~1tn trees without encroaching on current food production levels (although we'd have a fairly catastrophic impact on biodiversity). This would sequester around 100Gt of CO2. So far, we've emitted about 2800Gt of CO2, with around 950Gt remaining in the atmosphere (the rest is already in trees/oceans etc).

So at the moment we could sequester all of the excess CO2 that's still up there in the atmosphere, in exchange for significantly altering the natural world.

However, planting 1tn trees takes time, and trees take a very long time to grow and sequester carbon. Once they're fully matured, they stop sequestration. While they're growing, humans would continue emitting ~35Gt CO2 per year at current levels, with "business as usual" projections of ~100Gt per year in 2100 (in the word "4 degrees of warming" scenario).

So although planting lots of trees is a valuable tool in the carbon sequestration arsenal, we really need fairly radical decarbonisation alongside it, if it's to be anything more than buying a decade at the end of the century.