Hi,
For internal usage within the company there are two options:
- Power BI Pro license, this is the $9.99 per month
- Office 365 E5
For your question, you don't need O365 to sign up for Power BI Pro
The sessions based model is for Power BI embedded and this is needed if you are using the Azure Power BI embedded solution th(is depends on the specific implementation)
Eyal