You might also find your answer in the LastPass Enterprise Manual.
What if I initially order a few Enterprise licenses, but want to purchase more later?
You are always free to purchase additional LastPass Enterprise licenses beyond your original purchase. When you order additional licenses, these new licenses are automatically synced to the same expiration date as your original purchase, and you receive a pro-rated price for the difference. This way, all your licenses will always have the same expiration date, and you don't need to worry about renewing at different times.