Base ampacity from NEC Table 310.16, corrected for ambient temperature and the number of current-carrying conductors sharing the raceway.
Usable ampacity is the base table value multiplied by two derates: the ambient correction √((Tcond − Tamb)/(Tcond − 30)) when the surroundings exceed 30 °C, and the bundling adjustment when more than three current-carrying conductors share a raceway (80% for 4–6, 70% for 7–9, 50% for 10–20). Even after derating from the 90 °C column, the final value may not exceed what the terminations allow — usually the 75 °C column per NEC 110.14(C).
You may start derating from it, but the result is capped by the termination rating — typically 75 °C for breakers and lugs. That is why the 75 °C column is the everyday working column.
A neutral carrying only the unbalanced current of a multiwire circuit generally does not count; a neutral serving line-to-neutral nonlinear loads (heavy electronics) does. See NEC 310.15(E).