How many DCs can you have in a cluster?

[Blair Zajac]

In this C* Summit 2013 talk titled "A Deep Dive Into How Cassandra 
Resolves Inconsistent Data" [1], Jason Brown of Netflix mentions that 
they have 5 data centers  in the same cluster, two in the US, one in 
Europe, one in Brazil and one in Asia (I'm going from memory now since I 
don't want to watch the video again).

Is there a practical limit on how many different data centers one can 
have in a single cluster?

Thanks,
Blair

[1] 
http://www.youtube.com/watch?v=VRZk-NhfX18&list=PLqcm6qE9lgKJzVvwHprow9h7KMpb5hcUU&index=57