how many years are in 1 millennium?

http://whatis.techtarget.com/definition/millennium

Click on the link above to see the answer.

I did I do not get it

Oh, I'm so sorry. What don't you understand about this first paragraph from that site?

"1) A millennium is a period of one thousand years. It is similar to the terms biennium , a period of two years, and century , a period of one hundred years. The term derives from the Latin mille , meaning thousand, and annum , meaning year."

But you didn't tell me what you don't get in that paragraph.

To determine the number of years in a millennium, we need to know the definition of a millennium. A millennium is commonly defined as a period of one thousand years.

To calculate the number of years in a millennium, we multiply the number of years in a millennium by 1. Using this approach, we find that there are 1,000 years in 1 millennium.

So, the answer to your question is that there are 1,000 years in 1 millennium.