I once back-of-napkin-calculated that these things should easily be able to contain a crew of 2 billion, just in terms of space. That's after taking into consideration that the engines and other such systems may take up a great deal of additional of space that's not the case in real warships.
Of course, the official crew numbers are something in the area of 200.000. But I prefer my numbers, they just seem more 40k.