It is possible to migrate only customers using migration tool.
But to achieve the same you need to know which tables are required to be migrated.
so, to break the solution into major steps :
step 1 : Comment all the non required steps
step 2 : Now Solve the errors for un-mapped fields if any.
step 3 : start migration by using cli command
bin/magento migrate:data <config.xml_path>
this should do the trick.
Get the idea about how tool is working and you will be able to perform above mentioned steps.
Now you asked about the steps in your comment, So, trying to answer that :
<step title="Customer Attributes Step">
keep this step and comment others. Solve the errors if any and run the data migration tool.