  The transition from a few-body system to a many-body system can result in new
length scales, novel collective phenomena or even in a phase transition. Such a
threshold behavior was shown for example in 4He droplets, where 4He turns into
a superfluid for a specific number of particles [1]. A particularly interesting
question in this context is at which point a few-body theory can be substituted
by a mean field model, i. e. where the discrete number of particles can be
treated as a continuous quantity. Such a transition from two non-interacting
fermionic particles to a Fermi sea was demonstrated recently [2]. In this
letter, we study a similar crossover to a many-body regime based on
ultralong-range Rydberg molecules [3] forming a model system with binary
interactions.
