Sheer numbers and military strength. I had 8500 EMS without DLC. It would`ve been possible.
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Okay, fair point. It is technically possible to get X EMS in-game, where X is a large number of your choosing.
You see Bioware, this is why you ought to have given the player an upper limit on preparedness for the final battle, not a constantly increasing number. Because if you want that battle to be a hard fought, last stand style fight, you shouldn't be providing the player with potentially unlimited military strength (at least as far as the number is concerned).
Putting aside the issue of EMS, I'm fairly sure that if you lined up ALL the forces of the galaxy against ALL the Reapers, we'd still lose. I imagine we'd make a dent, but nothing more than a dent. (What's the largest number of Sovereign class battleships seen on screen at any one time? Okay, compare that to an optimistic count on the number of dreadnoughts we have. I don't know what the answer is here, but since we'd need about 4 times as many dreadnoughts as them, I don't think we're going to come out on top.)