Hello @Ajaz Khan ,
If you wish to add additional space you need to purchase more storage which is available on the on the Site Collections page inside SharePoint admin or on the subscriptions page inside Office 365 Admin.
Normally, we do not recommend that you allocate 1TB of space for each Site Collection. Because max storage per site (site collection) is 25 TB(Please note that this is a site limit, not a tenant limit).
Please see "SharePoint limits" to learn about the service limits in SharePoint for Microsoft 365.
Thanks,
Echo Du
================================
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.