  Divacancy spins in silicon carbide implement qubits with outstanding
characteristics and capabilities, including but not limited to, 64 ms coherence
time, spin-to-photon interfacing, and sizable, $10\% -30\%$ room-temperature
read-out contrast, all of these in an industrial semiconductor host. Despite
these great demonstrations, there are still numerous open questions on the
physics of divacancy point defects. In particular, spin relaxation, which sets
the fundamental limit for the spin coherence time, has not been thoroughly
studied yet. Here, we carry out theoretical simulations of environmental spin
induced spin relaxation processes of different divacancy configurations in
4H-SiC. We reveal magnetic field values where the longitudinal spin relaxation
time T$_1$ drops resonantly due to the coupling to either $^{29}$Si and
$^{13}$C nuclear spins or electron spins associated with other defects and
dopants. We quantitatively analyze the dependence of the T$_1$ time on the
concentration of the defect spins and the applied magnetic field in the most
relevant cases and provide a simple analytical expression allowing either for
estimation of the T$_1$ time in samples with known spin defect concentration or
for estimation of the local spin defect concentration of an ensemble or a
single divacancy qubit from the measured relaxation rates.
