  Rare giant low surface brightness galaxies (gLSBGs) act as a stress test for
the current galaxy formation paradigm. To answer the question `How rare are
they?' we estimate their volume density in the local Universe. A visual
inspection of 120~sq.~deg. covered by deep Subaru Hyper Suprime-Cam data was
performed independently by four team members. We detected 42 giant disky
systems (30 of them isolated) at $z\leq0.1$ with either $g$-band
27.7~mag~arcsec$^{-2}$ isophotal radius or four disc scalelengths $4h \geq
50$~kpc, 37 of which (including 25 isolated) had low central surface brightness
($\mu_{0,g}\ge 22.7$ mag~arcsec$^{-2}$). This corresponds to volume densities
of 4.70$\times 10^{-5}$ Mpc$^{-3}$ for all galaxies with giant extended discs
and 4.04$\times 10^{-5}$ Mpc$^{-3}$ for gLSBGs, which converts to $\sim $12,700
such galaxies in the entire sky out to $z<0.1$. These estimates agree well with
the result of the EAGLE cosmological hydrodynamical simulation. Giant disky
galaxies represent the large-size end of the volume density distribution of
normal-sized spirals, suggesting the non-exceptional nature of giant discs. We
observe a high active galactic nucleus fraction among the newly found gLSBGs.
