Explanation:
It's reasonable to consider to set one DBWn process for each latch when setting multiple LRU latches in your initialization parameter file. The LRU (least-recently-used) latch controls the server process's access to write new buffers into the buffer cache. Oracle automatically sets the number of LRU latches to be one-half the number of CPUs on the system. When your host machine has only one processor, one LRU latch is sufficient. Contention for the LRU latch can impede database performance when a large number of CPUs are available on the host system, when only one DBW0 process is available on the database, or when the load between multiple DBWn processes is unequally distributed.
Incorrect Answers:

1: S latch should have no less than 50 buffers in its set.
3: There is no direct relation between shared servers and latches in Oracle.
4: Each latch requires at least one, not two, DBWn process.
Oracle 8, DBA Certification Exam Guide, Jason S. Couchman, p. 855-856. Chapter 20: Tuning Other Areas of the Oracle Database

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