  A search is presented for new phenomena in events characterised by high jet
multiplicity, no leptons (electrons or muons), and four or more jets
originating from the fragmentation of $b$-quarks ($b$-jets). The search uses
139 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV proton-proton collision data collected by
the ATLAS experiment at the Large Hadron Collider during Run 2. The dominant
Standard Model background originates from multijet production and is estimated
using a data-driven technique based on an extrapolation from events with low
$b$-jet multiplicity to the high $b$-jet multiplicities used in the search. No
significant excess over the Standard Model expectation is observed and 95%
confidence-level limits that constrain simplified models of R-parity-violating
supersymmetry are determined. The exclusion limits reach 950 GeV in top-squark
mass in the models considered.
