Some years ago the main online poker sites changed their software to allow players to be able to play more tables. This made it more profitable for the sites and also more profitable for good players because they could get more hands per hour against the bad players.
So, multi-tabling became, and still is, a very popular way to increase your profit potential. However it has to be noted that playing more tables means that you will not be able to watch each extra table as closely as what you could watch one on its own.
This means that you need to sacrifice or slightly sacrifice some quality and replace it with quantity. This delicate balance is what makes multi-tabling profitable. There are certain players that can play ten, twelve, sixteen and even twenty tables at the same time. To be able to do this requires a different style of play compared to if you were only playing one table. I often play as many as eight tables and the most that I have played is twelve. When I play a single table then I play more hands per hour per table and I see more flops.
However, if your goal is to play more tables with a simplified style then you should really be seeing fewer flops and not more flops. The game becomes much more complex post-flop and if you are playing on, say, ten tables and having to play post-flop on half of them at the same time then you are going to start missing profitable spots. This is acceptable and some simple stats reveal why that is the case. Let us say that you could make 20bb/100 hands playing one table and you see 100 hands per hour.
Clearly here then your hourly rate is 20bb/hour. Now let us say that you play six tables and see 600 hands per hour but your earn rate is halved and is only 10bb/100 because you are missing a lot more information. Because you are seeing six times as many hands per hour then this quantity offsets the slightly decreased quality. Your hourly rate is now 60bb/hour and is ultimately three times more than what it had been when you were playing only one table. It is never going to be six times more because of the fact that you will miss vital spots.
Other negative game factors will impact on your earn rate as well like timing out on other tables when you have critical decisions to make elsewhere and not being able to watch your opponents as closely.
So, it is clear that multi-tabling requires a different strategy to playing than a single table. As a rule then, tighter play is better when you start to add tables and the biggest factor is your seat at a table rather than trying to totally optimise your decisions.
However, it is clear that in order to play many tables at the same time, you either have to have a very big edge against the stake level or you have to be a world class player.