It really doesn't matter AT ALL how many Subs the Germans BUILT (and a lot were not completed at the end of the war and shouldn't be counted anyway), but how many they had OPERATIONAL and, even more important than THAT, how many they could keep AT SEA AT THE ONE TIME.
The info above is indicative ... even by the December 1941 when they 'had' 250 U-Boats, only 88 (a bit over 1/3) were OPERATIONAL ... and, of those, they could only keep and average of TWENTY FIVE at sea at any given time. Two Groups, if you stretch the definition.
June 42 - 336 Subs, 126 Frontboote - 59 at sea at any given time, on average, about 4 Groups.
December 42 - 403 Subs, 204 Frontboote - 97 at sea at any given time, on average, or 6 groups.
June 43 - 428 Subs, 214 Frontboote - 86 at sea at any given time, on average, 5-6 Groups. Tonnage sunk per U-Boat per day at sea sinks to less than 100 tons.
December 43 - 452 Subs, 175 Frontboote - 86 at sea at any given time, on average, still 5-6 Groups. Tonnage sunk per U-Boat per day at sea sinks to under 50 tons.
June 44 - 435 Subs, 178 Frontboote - 47 at sea at any given time, on average. Now down to 3 Groups. Tonnage sunk per U-Boat per day at sea remains under 50 tons.
December 44 - 428 Subs, 135 Frontboote - 51 at sea at any given time, on average. Still ~3 Groups. Tonnage now under 25 tons sunk per U-Boat per day at sea.
So, even at their maximum, 250 Frontboote, or 16-17 Groups, they couldn't keep more than SIX Groups AT SEA, on average, at any given time.
Note that, yes, they built a LOT - which is really irrelevant, but they LOST a lot and those that they built sank less and less ... and yet they never had more that 250 OPERATIONAL boats at any given time.
So, at the very least, the range and endurance of the U-Boats might need to be cut dramatically to represent both the number operational and the number that could be kept at sea.
The existing situation is suitable for german fanboys only.
I suspect that one other problem is that now German Subs start at 1940 technology ... yet the teething problems of the boats, torpedoes and detection systems they had at the beginning of the war were such that, even if that were true (and it certainly doesn't seem to be on any reasonable reading of the situation) they were certainly not OPERATING at that level of effectiveness IN 1939. And the Allies caught up with ASW and Escort effectiveness VERY quickly ...
Author, Space Opera (FGU); RBB #1 (FASA); Road to Armageddon; Farm, Forge and Steam; Orbis Mundi; Displaced (PGD)