What is the minimum number of seismic stations needed to determine the location of an earthquakes epicenter?

At least three seismic stations

Explanation:

To determine an earthquake epicenter requires coordination between at least three seismographs. This is because a seismograph is only capable of registering the strength and amplitude of an earthquake; that is to a say, a single seismograph can register the occurrence of an earthquake. This describes a circle with the seismograph at the center. A second seismograph, which can be hundreds or thousands of miles away, can generate a similar plot. The circles from these two stations cross each other at two points. Finally, the third station points to the epicenter as the only place where all three circles intersect.