  We study the breakpoint region on the outermost branch of current-voltage
characteristics of the stacks with different number of intrinsic Josephson
junctions. We show that at periodic boundary conditions the breakpoint region
is absent for stacks with even number of junctions. For stacks with odd number
of junctions and for stacks with nonperiodic boundary conditions the breakpoint
current is increased with number of junctions and saturated at the value
corresponding to the periodic boundary conditions. The region of saturation and
the saturated value depend on the coupling between junctions. We explain the
results by the parametric resonance at the breakpoint and excitation of the
longitudinal plasma wave by the Josephson oscillations. A way for the
diagnostics of the junctions in the stack is proposed.
