In this contribution we present new predictions on number counts of extragalactic radio sources at frequencies >5GHz, by extrapolating observational data coming from surveys at low radio frequencies. Our approach is somewhat different than before, because we provide a statistical characterization of the source spectral behaviours at high radio frequencies that takes into account, at least partially, the main physical mechanisms responsible of the emission. We focus on flat-spectrum radio sources, i.e. the dominant source population in the flux and frequency ranges we are interested in. Models of synchrotron emission from inhomogeneous unresolved relativistic jets predict that, at a particular frequency, jet emissions cease to be dominated by optically-thick synchrotron, and become dominated by synchrotron in optically-thin regime. Spectra then change its slope from ''flat'' to ''steep''. We determine this frequency on the basis of standard models for synchrotron emission in blazars and of typical values for the involved physical parameters. Moreover, we also discuss the contribution from ""inverted-spectrum"" sources, by using a purely statistical approach. We find that predictions of our model on number counts are in very good agreement with observational data, that cover a large frequency range up to ~220 GHz. Moreover, the model seems able to explain well the average steepening currently observed in the spectral slope of flat-spectrum sources at frequencies >~70