In general: as few as you can.
An overwhelming majority of the libraries that one can enable via build_visit are used for a single purpose: supporting a file reader ("database") to allow reading data of that type. If you don't select a particular library in build_visit, the only drawback is that you won't be able to use VisIt to read data files which were written using that library.
VisIt supports many file formats which are used across numerous scientific disciplines. As a user in one or two of those scientific disciplines, it's likely that you'll only use a few of those databases in your daily visualization needs, and unheard of that you would need all of them. To put things in perspective: VisIt supports upwards of a hundred different file formats. Many of those formats are only applicable to a specific group within a scientific domain.
Finally, while we make every effort to streamline the build_visit process, there are simply too many platforms and too many third party libraries to do exhaustive testing. By only enabling what you need, you're less likely to encounter build failures in build_visit—it can't fail to build if you don't try to build it!
A good rule of thumb is: if you haven't heard of a library, you don't need to enable it.>