how many databases can be backed up by default for a Single SQL server

There is no single default limit that allows backing up all databases on one SQL Server instance at once.

For the error pattern shown (Cannot create worker thread during many VDI/BACKUP DATABASE operations), the relevant documented limit is for snapshot-style backups of many databases at the same time:

• SQL Server uses five threads in Sqlservr.exe for the snapshot backup of each database.
• If many databases are backed up simultaneously, available worker threads can be exhausted.
• Microsoft recommends creating a snapshot backup of fewer than 35 databases at the same time.
• For Transact-SQL snapshot backup, the maximum number of databases in that feature is 64. If there are more than 64 databases on the server, SQL Server returns Msg 925 ... The maximum allowed is 64.

Given a server with about 920–940 databases, the failure is consistent with too many databases being processed concurrently, which can lead to Cannot create worker thread and follow-on backup failures.

For third-party backup products that use VDI/VSS, check these items:

1. Verify the SQL login NT SERVICE\SQLWriter exists and is in the sysadmin server role on the instance being backed up.
2. Verify the SQL Server VSS Writer service is started and its startup account is Local System.
3. Run VSSADMIN LIST WRITERS from an elevated command prompt and confirm SqlServerWriter is present and in the Stable state.
4. Reduce the number of databases backed up at the same time.

If the backup method is Azure VM SQL snapshot backup, the documented concurrent backup limit is 20 databases per VM.

• Troubleshoot SQL Server backup and restore operations
• Error when you create a snapshot backup of many databases at the same time in SQL Server
• Create a Transact-SQL snapshot backup
• Support matrix for SQL Server database and instance snapshot (preview) backups in Azure VMs