[marand]

I usually play 4 if I can find that many "good" tables and don't feel tired. (I play 10/20 6-max and I'm very picky with my table selection.) I have experimented with 5 and 6 tables, but I feel that I play much worse and I get tired very quickly.
I try to get some "real" reads, but very often I just have statistics on most of the players and really only notice very strange plays. I don't have time to observe how the opponents play against each other, but try to remember and write down how they play against me.

I think there are basically two ways to think about this:
1) Play very few tables, perhaps 1 or 2. Try to get as good reads as possible. Perhaps you improve your poker skills faster this way.
2) Play more tables with slightly lower winrate for each table, but you make more $/hr total. Try not to play on autopilot too much and try to improve your poker skills away from the table. For example by writing down difficult hands and analysing them later, posting at 2+2 and reading books.

I think both 1) and 2) are perfectly acceptable options and it really depends on what you prefer. For example some players would get very bored with 1) because of the few hands/hour, others would get very bored with 2) because it feels like they are playing like a robot. I think 1) is more common for those playing high limits and 2) for those playing middle limits or small stakes.