There is no fixed amount of tar that a tree produces. Tar yield depends on species, resin content, part of the tree, age, dryness, preparation, and the method used to heat it.
A living tree does not usually produce tar in the way it produces sap or resin. Tar is made when wood, bark, roots, or other plant material is heated in low-oxygen conditions. So the question is really how much tar can be obtained from a certain amount of suitable tree material.
The answer varies widely. Resin-rich pine roots and stumps can yield more useful tar than ordinary clean trunk wood. Birch bark can produce tar on a small adhesive scale. Damp, poor, or unsuitable material may give little useful product and much smoke or residue. Equipment and skill matter as much as the tree.
This is why old tar-burning traditions often focused on particular materials. The maker did not simply cut any tree and expect a good result. For pine root tar, resin concentration in roots and stumps was part of the value. For birch tar, bark mattered.
A precise number would be misleading without specifying species, weight, moisture, method, and desired quality. The honest answer is that tar yield is variable, and historical producers learned through local experience which material was worth the labour.