Mark and Jonathan
It can use any number, three is probably a reasonable minimum, I don't know what the maximum is.2009/7/14 <firstname.lastname@example.org>or how many servers is an ideal setup say for a game 10?
If you have a small number of servers, there is probably no reason to use Cassandra; you can more usefully use a conventional database with replication or sharding.
The reason you might want to use Cassandra is to scale writes a lot, to levels you couldn't reach with more "conventional" databases.
In all likelihood, an application would use Cassandra to store high-volume high-write data alongside a more conventional database to store smaller, less frequently changing stuff.
It appears to me that Cassandra 0.3 is only really useful if running on a homogenous cluster of dedicated servers with reasonable amounts of memory / storage (But not necessarily high spec servers of CPU, IO speed or internal redundancy).
Once the load balancing is implemented in 0.5 (according to current plans) it will become more useful for heterogenous environments.