SharePoint Online allocates an initial 10 GB of storage plus 500 MB for each user. This storage is pooled and available for allocation across multiple site collections. For example, an organization of 1,000 users by default would have a base of 510,000 MB (510 GB) of storage.
In addition, users can purchase more SharePoint Online storage in GB increments charged monthly, currently $2.50USD/GB/month.
Site Storage Quotas
The SharePoint Online service administrator can set the storage limits for site collections and sites created by users. The minimum storage allocated to a new site collection is 24 megabytes (MB). The maximum storage available for any site collection is up to 100 gigabytes (GB).
The maximum SharePoint Online storage available to a single company’s tenancy is 5 terabytes (TB).