Choosing Redis key names and limit on the number of keys

Choosing Redis key names and limit on the number of keys

I heard a lot about the blazing speed offered by Redis server and hence
thought of plugging it into one of my existing Rails applications running
on PostgreSQL as the database server.
My question is if I am having about 100000 users on my system and would
like to implement a followers/following pattern, I can use Redis' SET
datatype or so., but is it a good practice to have 100000 different keys
based on user. Is this the right approach to define keys in my current
scenario? If so, what is the limit on the number keys on a single instance
of redis.
Suggestions on better designing of keys are welcome.