  Thermoelectric transport involving an arbitrary number of terminals is
discussed in the presence of a magnetic field breaking time-reversal symmetry
within the linear response regime using the Landauer-B\"uttiker formalism. We
derive a universal bound on the Onsager coefficients that depends only on the
number of terminals. This bound implies bounds on the efficiency and on
efficiency at maximum power for heat engines and refrigerators. For isothermal
engines pumping particles and for absorption refrigerators these bounds become
independent even of the number of terminals. On a technical level, these
results follow from an original algebraic analysis of the asymmetry index of
doubly substochastic matrices and their Schur complements.
