Thanks Usha di.
In response to your question: "How do these people know that 70,000 plants still need to be found..."
These estimations are extrapolations, as far as I know.
They analyze sample plots/regions and extrapolate the species diversity in similar vegetation types and finally up to country or region level.
If you study the diversity of sample plots, for e.g. in Western Ghats, of known size, then you can estimate the diversity in the entire W.Ghats by extrapolating the available data, even if you don't study the other areas of W.Ghats. Similar method for other different bioregions/vegetation types such as E.Ghats, Himalaya, NE India etc. Then you can estimate the diversity in India :)
The actuals may be less or more, as you know.
Gurcharan ji may be able to throw more light on this.
Vijayasankar Raman
National Center for Natural Products Research
University of Mississippi