  We consider the dependences of the average number of interacting nucleons in
high energy heavy ion collisions on the impact parameter in two cases, when the
colliding nuclei have equal atomic weights, and when one nucleus is
significantly more heavy in comparison with the second one. We argue that in
the case of trigger of some rare event (say, $J/\psi$, or $\Upsilon$
production) the multiplicity of the secondaries can change several times for
minimum bias sample, but it should be stable in the case of central events.
