The optimal number of mappers depends on many variables: you need to take into account your database type, the hardware that is used for your database server, and the impact to other requests that your database needs to serve. There is no optimal number of mappers that works for all scenarios. Instead, you’re encouraged to experiment to find the optimal degree of parallelism for your environment and use case. It’s a good idea to start with a small number of mappers, slowly ramping up, rather than to start with a large number of mappers, working your way down.
When you run sqoop import with -m 1 option, 1 mapper will be launched and in case this parameter is specified, sqoop will run 4 mappers by default.