  We show that the number of horizons of static black holes are constrained a
lot by classic matter satisfied some energy conditions. With very general
assumptions, we prove that if the interior of a black hole is dominated by
classical matter, there is at most one non-degenerated inner horizon behind the
event horizon; in addition, if the event horizon is degenerated, there exists
no inner horizon. Our result offers some universal restrictions on the number
of horizons. Interestingly and importantly, it also suggests that classical
matter not only promotes the formation of event horizon but also prevents the
appearance of multiple horizons inside black holes. Furthermore, using the
geometrical construction, we obtain a radially conserved quantity which is
valid for general static spacetimes.
