  (Abridged) The properties of stellar clumps in star forming galaxies and
their evolution over the redshift range $2\lesssim z \lesssim 6$ are presented
and discussed in the context of the build-up of massive galaxies at early
cosmic times. We use HST/ACS images of galaxies with spectroscopic redshifts
from the VIMOS Ultra Deep Survey (VUDS) to identify clumps within a 20 kpc
radius. We find that the population of galaxies with more than one clump is
dominated by galaxies with two clumps, representing $\sim21-25$\% of the
population, while the fraction of galaxies with 3, or 4 and more, clumps is
8-11 and 7-9\%, respectively. The fraction of clumpy galaxies is in the range
$\sim35-55\%$ over $2<z<6$, increasing at higher redshifts, indicating that the
fraction of irregular galaxies remains high up to the highest redshifts. The
large and bright clumps (M$_{\star}\sim10^9$ up to $\sim10^{10}$M$_\odot$) are
found to reside predominantly in galaxies with two clumps. Smaller and lower
luminosity clumps ($\log_{10}\left(M_{\star}/\mathrm{M_\odot}\right)<9$) are
found in galaxies with three clumps or more. We interpret these results as
evidence for two different modes of clump formation working in parallel. The
small low luminosity clumps are likely the result of disc fragmentation, with
violent disc instabilities (VDI) forming several long-lived clumps in-situ, as
suggested from simulations. A fraction of these clumps is also likely coming
from minor mergers. The clumps in the dominating population of galaxies with
two clumps are significantly more massive and have properties akin to those in
merging pairs observed at similar redshifts; they appear as more massive than
the most massive clumps observed in VDI numerical simulations.
