Quote: Maximum number of fields for one record
Theoretically - none. Limited by memory addressing and length of lists that may appear. A practical limit would be in the thousands.
Quote: Maximun number of records for one Database
Limited by memory addressing (2 Gig on Win32, 4 Gig on else). Practical limit is based on the speed and memory size of your server. In other words - grow till you get slow, then slap some RAM in and grow some more.
Quote: Maximum file size
2 Gig for either the .DAT or the .DB
Quote: Maximum number of databases open concurently
Theoretically unlimited. Practically, I would start to feel very uncomfortable approaching one hundred. Again, entirely based on the limits of your hardware and operating system. We do not keep files "open" during operation. They are opened and closed only as needed.
Quote: Type of indexing used (if any)
Internally we use linked lists with arbitrary memory position "nodes". No user-centric indexing of any kind.
Field length limit?
Again, theoretically - none. But practically it would be unwise to exceed 32768 bytes in a text field.