Explanation/Reference:
Note:
* To attach a content database to a web application by using Windows PowerShell Start the SharePoint 2013 Management Shell.
At the Windows PowerShell command prompt, type the following command and then press ENTER:
Mount-SPContentDatabase -Name DatabaseName -DatabaseServerServerName -WebApplication URL
* Attach the remaining databases (no need to wait until first command finishes and use new instance) After you restore the first content database and verify success, you can continue to restore and upgrade other databases. You can perform parallel database attach upgrades to upgrade more than one database at a time. Use separate Command Prompt windows (i.e new instance) to run multiple upgrades. It is recommended that you separate the start time for each new database upgrade session by several minutes to prevent issues with temporary locks set for the web application during attachment. Otherwise you might receive an error on the upgrade session. The wait time to clear temporary locks varies depending on the number of site collections, or the speed of the database server hardware.
Reference: Upgrade content databases to SharePoint