The maximum number of databases a user may select from the database hierarchy list is obtained via the following method in the SearchManager API:
public int getNumMaxSearchableDb()
However, this implementation was never really completed - it currently returns a hard-coded answer (eight) that is sufficient for the ExLibris and Sirsi metasearch products, but may not be correct for other metasearch engines.
Ideally, this method should find it's "answer" by calling out to the Citations configuration service. To support this, we'll need to update the configuration service to lookup this value in the XML configuration file.