What is the maximum allowable load capacitance for 74HC04?
There really isn't any. The greater the capacitance the slower the output transition, but at least for single outputs, the output can be short-circuited to ground indefinitely. If all outputs are shorted the chip will probably overheat. The slow transition produced by driving a large capacitance can cause big problems if the output is driving other logic unless a Schmitt trigger is used. So how slow a rise or fall time are you willing to accept?