How to ensure parallelism = 1 in kafka stream when input stream has more than one partition?

I am currently using akka and considering switching to kafka stream instead. In akka, when creating source, I can clearly state .mapAsync(1) to ensure the number of app running is just 1.
Checking the kafka documentation, my understanding is that by default, number of app running is set by number of partitions of the input topic the application is reading from in its processing topology. The only cofig I found is num.stream.threads that will set how many threads are running per app.
Is there any way to control parallelism and set it to 1 instead of max number of partitions?