Power BI (Free) vs Powwer BI Pro license difference

This really depends on what those other employees (is it 15,000 total or 1,500 total?) will be doing with Power BI. Most likely, you will want one or more Premium instances or Power BI otherwise if your Pro users publish something, the Free users will not be able to interact with the dashboards and reports. Another way to go would be to use Power BI Report Server on premises. See a couple of my articles here:

https://www.linkedin.com/pulse/power-bi-licensing-pictures-greg-deckler-microsoft-mvp-

https://www.linkedin.com/pulse/power-bi-tableau-licensing-deconstructed-compared-greg