They already built enormous number of U-boat. U-boat haven’t lost the battle for Atlantic because they were outnumbered. Allies developed tactics to effectively defeat them. Having more U-boat could only produce more sunken U-boat and killed crews.
They simply didn't have enough at one time; that was the problem.
Doenitz had argued vociferously for three hundred U-Boats in his arsenal before war with England could be considered. (He would have one hundred boats on station, one hundred coming and going, and one hundred in port being refitted. He considered the total number sizeable enough to cover the losses.)
(By the way, no historian familiar with the subject doubts that this would have led to Britain's suing for peace shortly after hostilities began; ship losses at the onset of the war as well as in the spring of 1942 after the Germans switched to the 4-rotor Enigma had a calamitous effect on food supplies in England. The country was on its back.)
At the onset of the war, Germany had 54 U-Boats, 5 of which were training vessels and not fit for combat. The country simply never had a U-Boot force at one time that was sizeable enough to interrupt the shipping to England to the degree necessary to impel England to throw in the towel. It became an endurance race, which it was only going to lose when the US officially took sides in the war. (We had been "unofficially" helping the Royal Navy pursue U-Boats for some time in the North Atlantic in 1941, but that's a subject for a different thread.)
Not early in the war before America entered and before the Royal Navy had developed effective countermeasures. More U-Boats early on could have forced Britain out of the war.