You might also find your answer in the LastPass Enterprise Manual.
How do I purchase or renew LastPass Enterprise?
Just getting started? Sign up for a Trial. To purchase LastPass Enterprise you must start with a trial. You can sign up for the trial at https://lastpass.com/enterprise_trial.php . You will be prompted to purchase licenses for Lastpass Enterprise at the end of your trial or you can purchase licenses immediately.
Purchase Licenses. If you would like to purchase initial or more licenses throughout the year, go to the Home Page of the Admin Console and clicking 'Purchase More Licenses!' Initial purchases will show the full price, any additional licenses purchased throughout the year will be at a prorated rate so that all licenses expire at the same time as your intitial purchase.
Note that only Enterprise Administrators can purchase more licenses. Direct link to the Admin Console: https://lastpass.com/enterprise_home.php