The premise of the question is incorrect, in that the Enigma was never "cracked". If you read "The Hut Six Story" by Gordon Welchman, you will find that Enigma messages could only be translated when operators made errors such as using the same key repeatedly or repeated use of the same base codes ("discriminants"). When the Enigma was used correctly it was unbreakable.
Nevertheless, many transmissions were intercepted and decrypted due to improper operational use of the machine.
The estimate of lives saved is based on the idea that the project shortened the war by "two to four" years, thus extrapolating that 2 years times 7 million deaths per year is 14 million. The origin of the "two to four" years idea I think is the 1974 book "The Ultra Secret" by Winterbotham.
While cryptography operations were certainly helpful, claiming that it shortened the war by two years is an overstatement. For example, in the Battle of Atlantic, the theater in which the decryptions were supposedly most important, I doubt anyone would characterize code breaking as the decisive technology. Radar, direction finding, convoy tactics, sonar, and anti-submarine aerial patrols, especially from Iceland, were all probably more important to the outcome than code breaking.