Finding maximum sink and source currents from datasheet
The values given for V(oh) and V(ol) state the current that the device can provide and still provide reliable logic levels. For a CMOS device you can estimate the max Rds(on) for the driver  FET pair and thus establish what the theoretical maximum current would be for the pin.
There’s also a max total current specified for all the pins in aggregate, and per port. See Note 6 in the DC Characteristics, in the datasheet here.