User blog comment:GodzillaMaster/Truly Extinct?/@comment-4466838-20111118151045/@comment-118213-20111118210847

Looking at the size of the cruiser compared with the collector base makes you think that their must have been more than one. Shepard and her crew also must have thought there was more than one since they found it significant that they ran into the same cruiser so many times; had they thought it the only one of it's kind they may not have seen the exact significance of running into that particular ship so many times.

As for further forces, the other collector cruisers would have likely been destroyed when the base went up, if not: those beyond the galactic core when the base went up would have been few in number and hardly viable for major ground forces attack; baring in mind the size of the colonies that a single cruiser attacked. I doubt we'll see the Collectors in ME3, but that is just my opinion.