  We study the efficiency of pair production in polar caps of young pulsars
under a variety of conditions to estimate the maximum possible multiplicity of
pair plasma in pulsar magnetospheres. We develop a semi-analytic model for
calculation of cascade multiplicity which allows efficient exploration of the
parameter space and corroborate it with direct numerical simulations. Pair
creation processes are considered separately from particle acceleration in
order to assess different factors affecting cascade efficiency, with
acceleration of primary particles described by recent self-consistent
non-stationary model of pair cascades. We argue that the most efficient
cascades operate in the curvature radiation/synchrotron regime, the maximum
multiplicity of pair plasma in pulsar magnetospheres is ~few x 10^5. The
multiplicity of pair plasma in magnetospheres of young energetic pulsars weakly
depends on the strength of the magnetic field and the radius of curvature of
magnetic field lines and has a stronger dependence on pulsar inclination angle.
This result questions assumptions about very high pair plasma multiplicity in
theories of pulsar wind nebulae.
