  Massive black holes are key components of the assembly and evolution of
cosmic structures and a number of surveys are currently on-going or planned to
probe the demographics of these objects and to gain insight into the relevant
physical processes. Pulsar Timing Arrays (PTAs) currently provide the only
means to observe gravitational radiation from massive black hole binary systems
with masses >10^7 solar masses. The whole cosmic population produces a
stochastic background that could be detectable with upcoming Pulsar Timing
Arrays. Sources sufficiently close and/or massive generate gravitational
radiation that significantly exceeds the level of the background and could be
individually resolved. We consider a wide range of massive black hole binary
assembly scenarios, we investigate the distribution of the main physical
parameters of the sources, such as masses and redshift, and explore the
consequences for Pulsar Timing Arrays observations. Depending on the specific
massive black hole population model, we estimate that on average at least one
resolvable source produces timing residuals in the range ~5-50 ns. Pulsar
Timing Arrays, and in particular the future Square Kilometre Array (SKA), can
plausibly detect these unique systems, although the events are likely to be
rare. These observations would naturally complement on the high-mass end of the
massive black hole distribution function future surveys carried out by the
Laser Interferometer Space Antenna (LISA)
