  The average number $S_N(t)$ of distinct sites visited up to time t by N
noninteracting random walkers all starting from the same origin in a disordered
fractal is considered. This quantity $S_N(t)$ is the result of a double
average: an average over random walks on a given lattice followed by an average
over different realizations of the lattice. We show for two-dimensional
percolation clusters at criticality (and conjecture for other stochastic
fractals) that the distribution of the survival probability over these
realizations is very broad in Euclidean space but very narrow in the chemical
or topological space. This allows us to adapt the formalism developed for
Euclidean and deterministic fractal lattices to the chemical language, and an
asymptotic series for $S_N(t)$ analogous to that found for the non-disordered
media is proposed here. The main term is equal to the number of sites (volume)
inside a ``hypersphere'' in the chemical space of radius $L [\ln (N)/c]^{1/v}$
where L is the root-mean-square chemical displacement of a single random
walker, and v and c determine how fast $1-\Gamma_t(\ell)$ (the probability that
a given site at chemical distance $\ell$ from the origin is visited by a single
random walker by time t) decays for large values of $\ell/L$:
$1-\Gamma_t(\ell)\sim \exp[-c(\ell/L)^v]$. The parameters appearing in the
first two asymptotic terms of $S_N(t)$ are estimated by numerical simulation
for the two-dimensional percolation cluster at criticality. The corresponding
theoretical predictions are compared with simulation data, and the agreement is
found to be very good.
