Hello,
PCA9450 integrates 6*DCCs and 5*LDOs, they work as one chip.
There are some methods to calculate the power Dissipation from each regulators to get the total power dissipation to get a ΔT with thermal resistors.
For DC-DC, Power Dissipation = Pinput * (1-efficiency);
For LDO, Power Dissipation = (Vin-Vout)*Iout;
then we can get the total power dissipation(P) of all the regulators, P*Rth(ja) = ΔT, Tambient + ΔT should be smaller than max Junction temperature (125℃)

Hope it helps,
Regards.