It only consumes a license when something calls the web service. While Blue Prism is not normally trigger based, web services and CLI commands are the exception. Sessions that are generated by either approach do not consume a license until the call is made. Each web service call will spin up a new session.
There's more explanation in the web services document: https://portal.blueprism.com/documents/standard?title=web+services&cate…
Dave Morris, 3Ci at Southern Company