The effects of LEO (low earth orbit) satellite orbit altitude, inclination and look-down antenna field of view on the number and distribution of specular reflection points in GNSS-R events were analyzed, based on GNSS-R mathematical criterion. Using the BDS, GPS, Galileo, and GLONASS data, we combined these 4-system constellations. The number of channels of GNSS-R receiver required was estimated. The results show that the number of GNSS-R events increase with the increase in the LEO orbit altitude and look-down antenna field of view. With a decrease of LEO satellite orbit inclination, the specular reflection points tend to distribute in the equatorial region. Meanwhile, the requested number of GNSS-R receiver channels increase with the increase of LEO orbit altitude and look-down antenna field of view, but might be affected slightly by the variation of LEO orbit inclination. The study provides a theoretical reference for future GNSS-R mission design.