I usually play 4 tables at once, 5 tops without losing play quality. What I definitely cannot do is to play both tourneys and cash games at the same time. Game play and strategy is so different that I easily get them confused during play.
I do remember when I started playing online and opening two tables back then was so challenging. I know of people playing 12 or more tournament tables at once, big buy-ins tournaments, and do pretty well.
It's all a matter of adjustment I think, and preference. If you want to try multitabling, then go for it. Just start one step at a time: 2 tables first, until you are comfortable to add 1 more, and then another one, etc. If you are not doing it already, start using a HUD as it helps A LOT when multitabling. It is much easier to look at a player stats at a glance before deciding your moves with a HUD. With just one table you can easily take notes and even remember other players actions and ranges. With two or more tables, that becomes very difficult and that's where the HUD becomes handy.
Finally, there is some help online regarding multitabling, so that might be worth a read to.
GL at the tables!