how many animals do scientists estimate are yet to be discovered?

The discovery of an unknown species is the center of many scientific reseaching being conducted right now. Especially in Germany and central Europe, where there are highly specialized reseach centers that place great importance on the biodiversity of possible new species but also the implications that would arise from such discoveries. Just recently a group of German scientists from the UFZ Center for Environmental Research discovered a new insect near the town of Dessau, Germany. It is an extremely rare hover fly, of which only six specimens are known up to now. As new scientific techniques and technological advancments arise, we can expect many more discoveries of new species in the future.

You pose an unanswerable question since you cannot estimate the undiscovered. Now if you wanted an estimate from one scientist then just asking them would give you an answer but as for science there would be probabilities based on past trends but as they would be WAGs ( WILD ASSED GUESSES ) then they are not going to be labeled as 'estimates'.