To list all databases of a Redis instance (i.e. version 2.2) the following command yields the desired information.
$ redis-cli INFO | grep db db0:keys=1500,expires=2 db1:keys=200000,expires=5 db2:keys=350003,expires=15
If you're just interested in the amount of currently existing databases (which is limited to 16 per default but can be increased by altering redis.conf) pipe it further to word count.
$ redis-cli INFO | grep db | wc -l 3
Inside a redis-cli instance you can also issue the
INFO command with the optional
redis> INFO keyspace # Keyspace db0:keys=8577,expires=0
In newer versions (i.e. 2.6.2) you can even just call
redis-cli INFO KEYSPACE on the console which gives you also the desired information.