  Flat Spectrum Radio Quasars (FSRQs) are the most powerful relativistic jets
seen from supermassive black holes (BHs) accreting via a radiatively efficient
thin disc. Their high energy emission is well modelled by highly relativistic
electrons in the jet Compton upscattering an external source of seed photons,
primarily from the broad line region. Strong Doppler boosting by the jet bulk
motion makes these FSRQs readily detectable by the Fermi Large Area Telescope.
We combine jet spectral models with scaling relations for the jet physical
parameters as a function of mass and accretion rate. This does not match well
to the Gamma-ray loud Narrow Line Seyfert 1s, assuming their low BH masses are
reliable, but is able to predict much of the spectral evolution observed along
the Blazar sequence. We use these models in conjunction with cosmological
simulations of efficiently accreting BH number densities, and find that they
overpredict the observed number of FSRQs by 2 orders of magnitude if all of
these objects produce a FSRQ jet. We can better reproduce the observed numbers
if jets are only produced by high spin BHs and BH spin is built from
chaotically aligned accretion episodes so that high spin BHs are rare. However,
this does not reproduce the observed redshift and mass accretion rate
distributions of the FSRQs. This may indicate a redshift dependence in
accretion mode, with sustained alignment accretion episodes being more
prevalent at higher redshift, or that there is some other trigger for FSRQ
jets.
