Salesforce Streaming API is unlimited to use according to Heroku Connect Now Provides Free Unlimited Salesforce Streaming API Events. Can I use it for all of my mappings?
The number of streaming events is unlimited, but each Mapping that uses Accelerated Polling creates and uses a PushTopic in Salesforce. Salesforce still limits the number of PushTopics in an organization. You should be aware of this limit and the impact that enabling Accelerated Polling may have.