104th Annual Meeting of the American-Urological-Association, Illinois, Amerika Birleşik Devletleri, 25 - 30 Nisan 2009, cilt.181, ss.80, (Özet Bildiri)
We collect some recent results that together provide an almost complete answer to the question stated in the title. For the dimension d=2 the answer is three. For the dimensions d=3 and d>4 the answer is four. For the dimension d=4 the answer is either three or four. Curiously, the exact number in d=4 seems to be an open problem.