You got it correct. Each partition will allow 1 task to subscribe. A single task can subscribe to many partitions, but one partition can have 1 task for the same connector.

Whether there are different topics or not, only total number of partitions should matter. For your case, 17 tasks for 17 partitions is what you needed to do.

How did you check if they are all assigned to task-0? If you are sure about that, there maybe a consumer assigning issue with your custom connector.