Not as much as I would have thought.

2TB can be one server: http://www.supermicro.co.uk/products/system/5U/5086/SYS-5086...

An expensive server for sure... but it's just one server.

As others have said they don't actually have a lot of RAM dedicated to Redis. You can put 1.5TB of memory in many 'inexpensive' Dell servers (single machines, not clusters). So basically a cabinet of machines you could have over 30TB of memory available to you. Basically some of the design choices Twitter has made seem tailored to their (previous? Not sure if they have their own hardware now) choices to run on 'cloud' services in the past.

With good hardware and a bit of a budget you can easily scale to crazy numbers of processor cores and memory. That isn't to say the software side of the solution is going to be any easier to solve.

That's really not a lot, especially when you consider that it's distributed among a cluster.

A single SPARC M5-32 can have up to 32TB of memory:

http://www.oracle.com/us/products/servers-storage/servers/sp...

Even the low end of single servers goes up to a couple TB of RAM. "Enterprise" hardware has been there for ages.