There currently are 24 operational satellites as part of the GPS satellite constellation, that orbit the earth with a orbital radius of ~16,000 miles (from the center of the earth). The satellite orbits are spaced out in 6 orbital planes (orbital plane is the plane surface formed by the path traced by the orbit of the satellite – it is nearly circular). The six orbital planes are separated by an angle of 60 degrees. Refer to the constellation figure below. Four to six satellites occupy each orbital plane.
In addition to the 24 operational satellites, there currently are 6 satellites in orbit that are maintained as spares, should any of the 24 operational satellites malfunction. The six spare satellites are kept operational for users to receive signals from them just like from the 24 satellites. However, the U.S. Government does not guarantee their availability at all times.
Remember that the GPS satellites are being launched since 1974. Satellites like everything else have limited lifespan. The GPS satellites have been designed to provide a lifespan of ~8 years. New satellites are being launched to space at regular intervals of time to make up for satellites that have lived out their lifespan.
When the satellites are launched into space, they are inserted to pre-calculated positions in their orbital planes. The pre-calculations ascertain that a GPS receiver located anywhere on earth and at all times, can receive signals from at least four satellites. This is required because earth blocks the GPS signals from passing through it. For a GPS receiver to be able to receive a signal from a satellite, the straight line-of-sight line should not have any obstructions in between. Since the satellites are flying around the earth, not all satellites are visible to a GPS receiver located somewhere on earth. Thus, though there are 30 operational satellites, not all of them can be visible at a location on earth, at any given time.
For a more comprehensive knowledge of GPS today, refer to websites such as www.gadgetknowledge.com/gps1.html