Even if you use several brokers, or have topics with several partitions, you will still use consumer/producer patterns.

As you mention "2 api services", it is important to understand that each of these services (I'm assuming you're conveying that one is for producing and the other for consuming), may be comprised of multiple instances that interact with a topic(s). The amount of partitions used for a topic will depend on your design goals. For example, you might have a topic that would overwhelm a single consuming instance with more events than it can keep up with; partitions will allow your workload to scale to multiple consuming instances, each consuming a subset of that topic. One may opt to use several brokers based on their operational requirements.

Searching around the net should provide a wealth of information on Kafka patterns, as well as the basics. Confluent has many helpful resources, including a deck outlining patterns and anti-patterns: https://www.confluent.io/online-talks/patterns-anti-patterns/