http://www.fao.org/docrep/W7261E/W7261e08.htm
It is estimated that some 10,000 plant species are used medicinally, most of these are used in traditional systems of medicine. However, only a relatively small number of species are used in any significant volume. For example, in TCM, 9,905 botanical materials are used but only an estimated 500 are commonly used. (Natural Medicine Marketing, 1996).
A study undertaken by the International Trade Centre (ITC) in 1982 suggested that 400 species were used in Europe (ITC-UNCTAD GATT, 1982). More recent findings suggest that the number could be closer to 1,500 including those used in homeopathy (Lange, 1996). There are no reliable data on the number of plant species that are currently traded in high volume; indeed, such a list is badly needed.
An analysis of plant derived materials used in prescription drugs during 1980 found that only 40 species of higher plants are used as sources of drugs. (Farnsworth et al., 1986)