The Arabic alphabet has 28 letters, and while some Surahs do contain a large number of letters, none contain all 28.
Perhaps you're thinking of the Surah Al-Fatiha, which is often referred to as the "mother of the Quran" due to its importance. However, it doesn't contain all the Arabic letters either.
If you're interested in learning more about specific letters or Surahs in the Quran, I'd be happy to help!