Thirteen. That's the number of database adapters that Sequel currently supports.
By comparison, ActiveRecord officially supports only 4 adapters, and ruby-DBI officially supports 5.
The only database supported by the other guys that Sequel doesn't support is SQLite 2, and that's only because it's so old that it wasn't in common use when Sequel was originally developed.
One reason that Sequel supports so many adapters is that the adapters are easy to write, only 5 methods are required.
The shortest adapter is OpenBase, which is only around 50 lines.