Hi Michael,
Are you using NI Volume License Manager? NI Volume License Manager 3.1 was recently released and it has some great new features that will help you answer these type of questions.
We have improved administrator's ability to see which licenses are in use with the new In-Use tab:
This gives you an accurate view of current usage for each license. In order to get the most up-to-date data, you can click the reload button.
We have also added Reports to VLM 3.1. The Seat Utilization reports are designed to answer your question of how many new licenses to acquire.
This screenshot shows the usage of a concurrent DIAdem license over a year. The report will show how many licenses were in use as well as displaying how many users were denied. This information will help you determine how many licenses you need.
If you are already using VLM 3.0, you can download NI VLM 3.1 and use your existing license file. If you are using an older version of VLM or FlexNet Publisher, contact agreements@ni.com and let them know you need a new license file.
Please let me know if you have any additional questions!