If one dozen people will stop eating beef, how much will this help to slow down global warming?

If one dozen people will stop eating beef, this will reduce the yearly global CO₂ emissions by around 48 tons CO₂-equivalent¹ (about one quarter of their total emissions, and half their emissions from food).

That’s the equivalent of planting about 48 trees per year, with the assumption of an old tree weighting 1 tonne, half of which is carbon. CO₂ mass is carbon mass times (44 / 12), but an average tree in a forest has less than that, because they don’t start as old trees, so you get roughly 1 ton of net CO₂ absorption from one newly planted tree.

So if you assume that the one dozen people will still live for 50 years without eating beef, they will have done as much good for the climate as if they had planted 2,400 trees. That’s roughly a forest with a size of 5 hectar (assuming 50,000 trees per km²).