Calculating the numbers of trees that need to be planted to offset one's carbon emissions is not as straightforward as it seems. It depends on the variety of trees planted, but also on the age of the trees, since older and bigger trees absorb more CO2 than saplings. A tree needed to survive for a number of years to calculate the average CO2 fixed during its lifespan.

Local climate and soil also has an influence as trees tend to grow faster in warm/hot and humid regions and on a fertile land. Tall lush trees in a tropical forests will fix more CO2 per year than small fruit trees, or even birches, in northern Europe, of course. In temperate climates, an oak tree grows much more slowly than a pine tree, so the pine will absorb CO2 more quickly in the first 2 or 3 decades, but won't live as long.

According to this 2018 scientific study the same species of trees sequester far more CO2 in tropical regions than in temperate or boreal ones. For coniferous trees the difference of carbon sequestration between boreal and tropical dry climate is nearly 10 times.

I have checked various websites to obtain estimations of the number of trees needed to fix 1 tonne of CO2 per year and that varies between 6 and 50 trees (6 being for fully grown trees, not newly planted ones). A reasonable average for European countries is around 20 to 30 trees. Let's say that 25 trees are needed to fix 1 tonne of CO2 per year in average, considering the whole lifespan of the trees.

If you are going to plant trees to offset your carbon footprint, it is far more efficient to plant them in tropical regions. I have compiled a list of carbon sequestration rate and cost efficiency by tree species based on data from Tree-Nation.