Hi there Nick, real great question you gave us! Thank you very much for such a solid and important doubt. I will try to help you, and the community, with my little experience:
Table Selection
First of all you gotta select the tables you are going to play in. When you select tables, choose those tables where you will be seat in a good position! (With TAGs and NITS to your left and with FISHES and WHALES to your right)
Playing with table selection will make you have position a lot of times upon weaker players (in the right) and because the blinds will fold too much you can steal and apply preflop pressure a ton, with raises, 3-bet lights, 4-bets light (Caution! See how much these regulars would fold to 3-bet first).
When you select one ideal table like this, play at least 15-20 minutes only with one table and then, open another. Unless, of course, you already have statistics and notes enough about the players. Try to have notes on all the tables you're involved in, and use colors to identify players tendencies and vices. Notes are crucial if you are playing in a deep pool.
My personal color plan goes like this:
No color: Average Regular
Yellow: Aggro donkey
Green: Passive Fish
Red: NIT
Orange: TAG
Blue: Decent LAG (Rare)
and so on

But these are mine, anyone be free to pick their own colors, for God's sake!
Once you open another table, do the same: focus only in this second table for 15-20 minutes (It could be 5-10 minutes, depend how fast you perceive player's images and styles). Once you dominated 2 tables (20 or 40 minutes) open the three and the fourth table. Of course you will not have a great volume of hands, if your session lasts one hour and a half or 2 hours, but once you open 4 tables, you don't have to worry about who you are playing with (because you have notes, colors, stats), which will make your decision making process, smoother.
Do not play multi-table if you are not prepared!
I don't have any shame to say that I play only 3 tables of 6-Max at maximum(4 tables of Full-Ring at maximum, but I stopped playing this for a while), or 1 table of Zoom. It increases my attention and my winrate. (1 table of Zoom = 3 regular tables).
Lemme try to illustrate some example: let's picture a professional chess player, a Game Master, precisely. This guy trained his mind in such a sharped form, that it can play with several opponents, by the same time and with a clock counting!
Now, why many poker players who are in the business for just a couple of years, have the ideia that they can "kill" multiple level thinking players, with the clock counting at the poker tables? Are we professionals or game master? I want to be, but I am not, yet.

Why do we have this divine capacity to play 4 tables at once (we are playing against 20 thinking humans beings at the same time!), with the clock running faster as a blitz chess game (a blitz chess game varies from 5 minutes to 15 minutes, I guess).
In poker online we have barely one minute in a half to think, and most of situation we take decisions right off the bat, without even blinking an eye, because certain spots had become our second nature.
And we see how long we take to analyse one simple situation in the turn: it takes hours to fully comprehend the thinking process of a hand. But when we are there, we have at maximum 2 minutes, if we think faster always, because of the "time bank", or whatever.

If you don't feel yourself comfortable playing many tables, play the ammount of tables that will allow you to have FULLY:
A) Focus
B) Emotional Control
Thus, you can achieve;
C) Decision making
Decision making is really what makes our winrate blows, however, the pressure is too high: besides, there is that whistle, alarm ticking, putting pressure on our minds to force us to make wrong choices!
Time should not be our problem. If we are having issues with time passing and we getting nervous with the alarm ticking, and then you have Pocket A's in one table and the Lag player raises you in the turn with a medium-connected board, plus you are in the BB with AKo and a NIT from the CO opens and a decent TAG from the SB 3-bets, by the same time there are two other tables running and UFF!!
I am satisfied opening just one Zoom table, because it increases my winrate a lot. I am not in a rush to make my roll grow fast and becomes a bank. I can work slow, learning every single spot, not ever fast folding just to watch another hand:
When I am playing fast-fold games (Zoom), I simply never fast-fold, as the game dictates: I fold and WATCH, so I can take notes of the players.

Okay, the field is highly preposterous, PokerStars 2 NLHE has a field of 1000 (one thousand players per day).
Of course when we go to 5 NLHE, we see a field of 500 players.
I already tried my friend, in the beginning I just opened 9 tables, 10 tables and pay a lot for it, because I couldn't assume to myself that I am unable to such a hard and shopisticated behavior, which is playing several tables at once.
It requires a lot of mechanical thought. There are many masters of multi-table Cash, for instance Nanonoko, from Brazil Claudio 'PeixeFeliz' Davino, Nathan 'blackrain79' Williams, etc.
I am sure I am not one of them. I don't feel bad about it! I feel good, because I know myself and I know what I can do and what I can't, what increases my winrate a little.
I hope it helps you buddy, I wish you a wonderful day!



Regards;
Carlos 'Aballinamion' Barbosa