Hi Hamid you are correct. The first one is a general statement of habit. The second would be more specific that you like to eat One egg, as opposed to two or more. You are emphasizing the number, as Francesco said.

Hello Hamid, that is right: the first option is what you would choose to say (and write). Here are some examples: "Yesterday I had an egg for breakfast"; "I would like a boiled egg, please''; "One egg for breakfast is all I need''; "I had two eggs for breakfast and one for lunch". As you can see, it is only when emphasising that it is only ONE egg (like in the third example) or when listing the number of eggs eaten (like in the fourth example) that we use the numeral 'one'; the rest of the time you would use the indeterminate article 'an' for a single egg, and no article for an unspecified number in the plural (or a number if you want to say how many).