Just want to add, that a table is not just a table. If we take a typical online MTT playing in either 9-max or 8-max format, it typically deal something like 50 hands per hour. Your VPIP in this format should be around 20% and definitely no higher than 25%. So there will only be 10 hands per hour or at most 12-13 hands per hour, where you have to do anything other than pressing the "fold" button, when you see your cards. This is not exactly a high workload, and this is also why, we see so many people being on their phone or doing other non-poker related things in live poker.
Yes in the ideal world we should pay attention to every little detail, when we are not involved in hands. But if we are playing online in micro or low stakes games against a bunch of donkeys and bad regs, thats quite a bit of an overkill, and nobody has the discipline to actually do it. And in top of that whats the point in achieving the absolutely best possible winrate in 3,3$ tournaments? If we can play 5 times as many of them with a slightly lower but still positive winrate, that will allow us to build our bankroll much faster and then move up.
However if we are talking about heads-up or shorthanded play, its a quite different story. Now we are involved in almost every hand, and the table might play 100 or 150 hands per hour, because there are less people, who need to make a decision preflop. So if you want to specialise in formats like heads-up SnGs or the popular Spin n Gos, which are typically 3-handed, then its totally reasonable to only play 1 or maybe 2 at a time.
Or if you play 9-man SnGs, it is a fine strategy to fire them up in "sets" of for instance 4. In that way you have 4 tables running, when you are playing with 9, 8, 7 or 6 people at the table, so that action is still slow, and you need to fold most hands preflop. But if you make it in the money, then usually there are only 1 or maybe 2 tables left, which allow you to fully focus on the action, when you need to.
And by the same token, if you have reached the final table of an MTT after 5 hours, then this is typically not the time, where you should be firing up more tables. Instead you should focus on the one, where the money now matter, at least to your bankroll. But by starting the session with for instance 4 MTTs rather than just 1, you vastly increase your chance of reaching that final table in the first place. And you avoid a situation, where it feel completely devastating to be the bubble boy, because now you literally spend 3+ hours for nothing.