How many TOP LEVEL SITES can possibly be created in a particular SharePoint admin center? Not under/below the main site ,but as a new top level site?
You can have upto 500,000 site collections in a single tenant as per the Msft Documentation.
You have one Site collection at root of your tenant i.e
All other sites collection under the managed path /Sites or /Teams.
For ON prem, their 500,000 personal sites and 250,000 other sites as supported limit. https://docs.microsoft.com/en-us/sharepoint/install/software-boundaries-and-limits-0
When you say top level, I'm assuming you mean Site Collections because otherwise, there is only 1 root site collection not matter if you're On-premise or Online
Below that you have managed paths
Therefore, the maximum value assigned to the number of site collections is not absolute, but is calculated based on an expected set of usage characteristics in which overall farm performance would be acceptable at the given limit under most circumstances.
Here is a handy guide I reference when wondering about SharePoint size and usage limitations:
As Waqas said, the maximum recommended number of site collections is 500000 in SharePoint online.