Go to general settings select user as a salesperson and select sales access rights as own documents only.

Add a salesperson in the allocate salespersons field in the customer form view.
You can allocate more than one salesperson to a single customer.

The salesperson can see only those customers, for which this salesperson is allocated.

The salesperson can create quotations/sales orders for only those customers, for which the salesperson is allocated.

Select the customers from the list view and then select mass update from the action menu.

After that mass update wizard will open, select update salesperson and add the user. Multiple customer salespersons are updated from here.

Multiple customers updated the salesperson by mass action.

You can also update allocated salespersons to multiple customers from the wizard.
You can update it by 2 methods, 'Add' and 'Replace'.
Add: It adds a new salesperson with the old salesperson.
Replace: It replaces new salespersons with old ones, so it removes old salespersons.
Here we update salespersons by using the 'Add' method.

Salespersons were added to the multiple customers by mass action.

Now we update salespersons by using the 'Replace' method.

Salespersons were replaced with multiple customers by mass action.