I have over 300 site collections with it's own content databases (1:1 relation. each site has it's own CD). Now while moving to spo 365 can I have it in the same architecture? I can't find any article on this.
2 Answers
SPO doesn't store documents in Content Databases but rather Azure Blob Storage. SPO is a radically different architecture from SharePoint Server in many ways.
While it is no longer possible to configure this, because SharePoint Server 2016 was a fork of the SPO code base at the time, I was able to get the SPO-style configuration set up.
Azure Blob Storage for SharePoint Documents
With regards to SPO, infrastructure is not your concern nor responsibility, which is a good thing!
With SharePoint Online you don't manage the Content Databases - it is a managed service.
So you would simply be migrating the Site Collections into SPO - and how the SQL databases are managed in the back end stops being your problem ;)