(Process mode only) Due to the operating system limit on the number of file descriptors per process on UNIX, reducing the number of engines reduces the number of network connections that the server can have. On Windows, the number of network connections is independent of the number of engines.
There is no way to migrate a network connection created for server-to-server remote procedure calls—or example, connections to Replication Server and XP Server—so you cannot take an engine offline that is managing one of these connections.