The free version or Tier provides only 2 partitions. Whereas with premium tier you can create a maximum of 32 partitions.
In the case of bidirectional communication, we dedicate a partition only for the sender to send the data, hence minimum is 2 partitions.
As the number of partitions increases, hence the cost increases, so choose partitions wisely, as once created they cannot be changed.
Maximum time only 4 partitions are used.
To read and learn about Azure IoT Hub. please see the following resources: