We present the space density evolution, from z = 1.5 up to z = 5.5, of the most massive (M => 10^9^ M_{sun}_) black holes hosted in jetted active galactic nuclei (AGNs). The analysis is based on a sample of 380 luminosity- selected ({lambda}L_1350_ => 10^46^ erg/s and P_5GHz_ => 10^27^ W/Hz) flat spectrum radio quasars (FSRQs) obtained from the Cosmic Lens All Sky Survey (CLASS). These sources are known to be face-on jetted AGNs (i.e. blazars) and can be exploited to infer the abundance of all the (misaligned) jetted AGNs, using a geometrical argument. We then compare the space density of the most massive supermassive black holes hosted in jetted AGNs with those present in the total population (mostly composed by non-jetted AGNs). We find that the space density has a peak at z ~ 3, which is significantly larger than the value observed in the total AGN population with similar optical/UV luminosities (z ~ 2.2), but not as extreme as the value previously inferred from X-ray-selected blazars (z ~> 4). The jetted fraction (jetted AGNs/total AGNs) is overall consistent with the estimates in the local Universe (10-20 per cent) and at high redshift, assuming Lorentz bulk factors {Gamma} ~= 5. Finally, we find a marginal decrease in the jetted fraction at high redshifts (by a factor of ~2). All these evidences point towards a different evolutionary path in the jetted AGNs compared to the total AGN population.