Welcome to the CO2 Calculator. This will allow you to see an estimate of how much CO2 the trees you plant will absorb over time.
It's an estimate because all trees are different. Some grow fast, some grow slow, some are just better at absorbing CO2 naturally. The amount they absorb also depends on the stage of life they're in. Generally speaking - the older they get the more they absorb. The 21kg per year is an average taking all of this into consideration.