Hi,
The 20 license minimum is there because of the cost to Microsoft of hosting two systems (D365 Customer Engagement and D365 Finance) to provide the full end to end experience included in D365 Project Operations.
If the customer is looking for multiple functionalities such as supply chain or finance, which might involve purchasing licenses for additional users, I believe these licenses would count towards the 20 user minimum, if that helps at all.
If the customer isn't looking to use the enterprise grade finance solution and instead would integrate with a different finance back end, they would be considering using the deal to pro-forma invoice, or so called 'lite' deployment:
docs.microsoft.com/.../determine-deployment-type
The 20 user minimum does still apply to the lite deployment approach but the community have created an 'idea' on the ideas portal that you can vote for, suggesting that Microsoft remove this restriction for 'lite' deployments.
experience.dynamics.com/.../
For now, the customer would simply have to purchase the minimum 20 user licenses in order to get access to D365 Project Operations.