We want to know the industry best-practice number of maximum databases
hosted on a single server with SQL Server, MySQL and Oracle. A link to
online source where this information is derived from is a real
advantage.

There is no "limit" for number of databases for SQL. It all depends on
your hardware and what type of preformance you want for each.

I can only speak for SQL Server where the limit is 32,767. As exms-ga
stated it does depend on hardware and not only disk access. I would
monitor each of your databases and try to achieve a constant flow of
disk access. If a database isn't needed for say 30-60 minutes you
should set the auto close flag to cut down on SQL Server overheads.