How much Program Memory does PIC24FJ1024GB610 have?
Don't confuse "memory space" (which is just the amount of all valid addresses, whether they are actually connected to program memory, RAM or some peripheral), and actual memory.

So, here, the memory space might be 8 MB, but, as the footnote says,


  exact boundary sizes are determined by the size of the implemented program memory.


So, some addresses in that space actually point to hardware memory, and others simply point to nowhere.