They only had 24 letters in their alphabet. And with their 24 letters they also could write down thousands and thousands of words. This is why the Lord called Himself two letters. He called Himself Alpha and Omega. The two letters He chose are the very first and last letters of the Greek alphabet.
Alpha is the first letter.
Omega is the last. When the Lord called Himself the Alpha and Omega, what He was saying was that He is, in a way, like the whole alphabet. Just as all the many, many words come from the letters of the alphabet, so, all the many, many things in the world come from Him. All the words come from the alphabet. All good things come from the Lord.