A MonetDB database instance manages a single database on a single system. It is perfectly possible to have multiple database instances run concurrently on a single system, but its resource claim may negatively affect the performance perceived.
All instances on a system are governed by a MonetDB deamon process. It provides a single point of access to any of the database instances locally, but also to those running in a local network. The deamon process ensures automatic (re) start of database instances when users request access, it can act as a proxy for clients, it can transparently handle fail-over and it can exploit data-parallel processing.