What happens if a consumer group has more consumers than partitions?

Each partition is consumed by exactly one consumer within a consumer group at any given time, so the parallelism of a group is limited by the number of partitions of the topic. You can start more consumers than partitions, but the extra consumers receive no partitions and sit idle (a common use is as standbys that take over when a member fails and the group rebalances). To get more consumer parallelism, increase the partition count, for example with kafka-topics.sh --alter --partitions; adding partitions does not redistribute existing data, and Kafka cannot reduce the count later. Share groups are a different kind of group: the number of consumers in a share group can exceed the number of partitions in a topic.

Expected key concepts: exactly one consumer, partition, consumer group, idle, no partitions, parallelism, rebalance, standbys, partition count, share groups