This subject was already discussed, this may help you :  http://markmail.org/message/6dybhww56bxvufzf#query:+page:1+mid:6dybhww56bxvufzf+state:results 

If you still got questions after reading this thread or some others about the same topic, do not hesitate asking again,

Alain

2012/3/19 A J <s5alye@gmail.com>
How many Column Families are one too many for Cassandra ?
I created a db with 5000 CFs (I can go into the reasons later) but the
latency seems to be very erratic now. Not sure if it is because of the
number of CFs.

Thanks.