Usually, poker rooms support 4-6 simultaneous tables at once, depending on the software. However, not all poker rooms support simultaneous view of all the tables you play in – in most multi-table supporting poker rooms, the table windows are overlapping each other, so you can see only one table at a time, which greatly reduces the possibility of implementing your skill with its full potential. Some poker rooms, however, support simultaneous multi-table view, with Poker Champs in the lead, while others require a very high screen resolution (usually at least 1600x1200) in order to view all the tables at once. It is strongly recommended to watch all the tables simultaneously - it makes a great difference in terms of profit as you are not forced to minimize and maximize different table windows constantly, thus you may direct significantly more attention on what is really going on on the tables.
If you think that you are skilled enough to play at multiple tables but haven't tried that yet, it is recommended to proceed through the tables gradually, i.e. you shouldn't take on 6 tables at once; try to play at two tables, then at three and so on.
However, playing at multiple tables may be dangerous, as you may lose a lot more if you do not focus on all of the games at once than when playing single-table games. Hence the greater bankroll swings and at least 50% greater bankroll required.