Unfortunately, there is no possibility to configure load distribution among Adaxes services. Still in the latest version of Adaxes Web interfaces distribute the load by automatically picking different service instances for operations. If you are using an older version of Adaxes, you can have multiple computers with both Adaxes service and Web interface components with a Web farm that will balance the load among Web interfaces. In this case, each Web interface will connect to the service instance on the same computer. For information on how to check the version of Adaxes you are using, have a look at the following help article: https://www.adaxes.com/help/CheckServiceVersion.
For your information, the load can also be due to scheduled tasks. Each of them is configured to run on a specific service instance first. As such, it can happen so that not only Web interface requests but also scheduled tasks are performed by a service. If you have a lot of scheduled tasks, it is recommended to have a dedicated service instance to run them. For information on how to configure a scheduled task to run on a specific Adaxes service, have a look at the following help article: https://www.adaxes.com/help/BindScheduledTaskToService.