The number of GPS satellites with global visibility at any location is shown in Fig. GPS Number of Visible Satellites. Different colors represent different levels of satellite visibility (See the legend on the right).
The computation of GNSS positioning requires three location parameters and one clock offset parameter, so at least four satellites need to be in view of the receiver simultaneously (positioning accuracy is also related to many other factors). The approximate coverage area of GPS is also illustrated in Fig. GPS Number of Visible Satellites.