Flink parallelism with Kafka source

[Guillermo Ortiz Fernández]

I'm looking for how Flink defines parallelism for a Kafka source (
https://nightlies.apache.org/flink/flink-docs-master/docs/connectors/table/kafka/).
How is it determined by default? Is it based on the number of partitions in
the topic? I have some topics with hundreds of partitions, and such a high
level of parallelism seems excessive.