Under very mild conditions, we prove that the number of components
in a decomposable logarithmic combinatorial structure has a distribution which
is close to Poisson in total variation. The conditions are satisfied for all
assemblies, multisets and selections in the logarithmic class.The error in the
Poisson approximation is shown under marginally more restrictive conditions to
be of exact order $O(1/\log n)$, by exhibiting the penultimate asymptotic
approximation; similar results have previously been obtained by Hwang [20],
under stronger assumptions.Our method is entirely probabilistic, and the
conditions can readily be verified in practice.