Recent experimental results (Giacconi et al, 79, Tananbaum et al 79) ascribe an increasingly important role to the contribution of discrete sources to the low-energy (few Kev) cosmic X-ray background (CXB). While the astrophysical nature of the objects involved is not yet clear, distant and powerful emitters like QSO play probably an important role (e.g. Setti and Woltjer 1979, Field 1980). For them, often the number-flux curve (LogN-LogS) provides useful hints on such properties as space distribution and/or evolution. For the case of the X-ray sources, moreover, a definite relation exists between their LogN-LogS and the granularity of the sky emission as described by the fluctuations of the X-ray background.