I have requirement that client is having thousands of database in SQL and thousands of application running on ASP.Net, now they want to move to SharePoint online. Is there a way to migrate all data and application on SharePoint Online?
This is a very broad question. This depends on what kind of data you would like to migrate to SharePoint online. There is SharePoint online migration tool which you can use to migrate data from on-premise site to SharePoint online but no specific tool from ASP.NET to SharePoint online. SharePoint online does not support farm solutions or sandbox solution so you need to see what kind of customization is present in existing application. There are lot of analysis required for each application and site. Then, based on that whether SharePoint online will be compatible for particular application and its data or not.
Thousands of DBs? No. Do not migrate this to SharePoint. Again: no. If they are trying to get off their local servers and into the cloud, then investigate running SQL in Azure VMs or Azure SQL. (or cosmos DB, or Azure Data Lake, etc). SharePoint is NOT a relational database, and the thought of attempting that migration is painful.
Also, do not try to re-write asp.net apps as to run in SharePoint. Again, if the goal is to get into the cloud, then investigate Azure Web Sites. You can take those existing ASP.Net sites, deploy them to Azure, and be up and running very quickly (most of the time, there are exceptions).
For the documents, yes, SharePoint is quite good at that, but even then, don't simply look to upload that much content without first spending time learning more about how SharePoint works so that you can organize the content effectively. Ramp up on sites, Office 365 Groups, Teams, permissions, Flow, metadata, managed metadata, site columns, and content types, as a start. Once you've done that, you'll be in a better position to look into that migration.
But, MS makes a SharePoint Migration Tool, and there are a number of 3rd party tools, including ShareGate.