  We present a theoretical study of entanglement in ensembles consisting of an
arbitrary number of particles. Multipartite entanglement criteria in terms of
observables are formulated for a fixed number of particles as well as for
systems with a fluctuating particle number. To access the quality of the
verified entanglement, the operational measure of the entanglement visibility
is introduced. As an example, we perform an analytical characterization of
quantum systems composed of interacting harmonic oscillators and witness the
entanglement via energy measurements. Our analysis shows that the detectable
entanglement decays for macroscopic particle numbers without the need for
decoherence processes and for all considered coupling regimes. We further study
thermal states of the given correlated system together with the temperature
dependence of entanglement.
