How to number tracks (in iTunes)?

I’ve taken up to edit my 9000+ tracks library. Now I’m wondering how is it with albums with two or more discs. Should the numbering of the tracks start at the beginning of the each disc or should it be continuous throughout the whole album?

Oh, and by the why, does anyone know why from time to time a message pops out in iTunes, saying: The iTunes Library File cannot be saved. you do not have enough access privileges for this operation (when I check the library file, the modification date is the same as the time as when the message occurs – apparently, the file has been accessed and written on, meaning iTunes did have access).

I’m using Windows 7, and have set Write permissions in the iTunes Media Folder for all users.