  We determine the flavour dependence of the renormalisation-group-invariant
running interaction through judicious use of both unquenched Dyson-Schwinger
equation and lattice results for QCD's gauge-sector two-point functions. An
important step is the introduction of a physical scale setting procedure that
enables a realistic expression of the effect of different numbers of active
quark flavours on the interaction. Using this running interaction in concert
with a well constrained class of dressed--gluon-quark vertices, we estimate the
critical number of active lighter-quarks above which dynamical chiral symmetry
breaking becomes impossible: $n_f^{\rm cr}\approx 9$; and hence in whose
neighbourhood QCD is plausibly a conformal theory.
