It is usually advised to play on more than 1 table at 1 go to make reasonable money on poker. On the other hand, playing on too many tables can result in lack of focus needed on each table. How many tables is it advisable to play on at 1 go, in order to have positive outcome? Is there a particular number of tables professional poker players play on at the same time?
Really there can't every be a fixed number or fixed answer for this. The simplest answer is what ever number you can play without having a negative effect on your game and your win rate.
Don't look to emulate some of the online wizards who grind 20+ tables, you will get lost and lose money. Start off playing one, open a second, if it goes fine and doesn't impact you, open a third, a fourth, etc, etc. The moment you find an extra table effects your game negatively, well then you've found your answer on the number of tables you can play.
The number of tables you can handle will never be static either. You'll find lower stakes you could play an extra few, while tougher levels will require fewer tables. Also the more you play and work on your game the more tables you'll likely be able to add too. So just keep that in mind.
I suggest finding the number of tables you can handle either with play money or at the micros.
It depends of the type of games you are playing and the familiarity you have with it as well. The difficulty of the decisions you have to make should also affect the number of tables you play. Also the time you have between each decisions will affect it. So at cash game in NLHE or tournaments or speed poker you will generally play a different number of tables. aAs you improve more and more decisions will be automatic and knowing what to do becomes almost like a reflex. Basically if you have time to do something else while you play (browsing forums or the internet in general, or anything else), it is generally time to open more tables.