What is the limit for polling configurations that a single instance can have?

A single polling configuration per instance is allowed on an object. If there are ten objects, then the polling configuration allows you to configure polling on all objects on the instance. An object cannot be configured twice on the same instance. 

While creating a polling object for each incident (incidents/1234/comments, incidents/5678/comments, etc.) how many polling objects can you configure? 

You have to account for every incident, by id, you want to monitor its comments. If there is a limit, only one incidentId per nested object(comments) is allowed.