If you are not sure whether N will ever be reached, then you don't need to deploy Cassandra until you reach a point where you're sure it will be reached.
2009/7/14 Johan Stuyts <email@example.com>
Is it unwise to use Cassandra in production if you use less than n servers? I.e. is it better to use another solution for <n servers and switch to Cassandra once n is reached?
If your application's scale is planned (i.e. by management who do planning-type things) to exceed what you can reasonably get out of a conventional database (with or without various types of scale-out solution), then Cassandra might be the right solution for you.
I feel that developing an application for Cassandra is a lot more difficult than a "traditional" database, and it's also a fairly immature product. There are many drawbacks, such as lack of visibility of storage usage.
I think that Cassandra is likely to become a very compelling system for large scale problems quite soon, but I don't think it's there yet.