The single most important factor is being a winner at the number of tables you're currently playing. If you're not a winning player, then there isn't even a discussion -- drop stakes or drop tables until you are. Then add slowly, maintaining a profitable rate before you add more.
I rather would play more tables - its like a variance reducer, or not ?
No. It just makes variance come quicker. Some will argue that multiple tables increases variance a bit. I'm not necessarily on board with that, but it largely comes down to what you define as variance. In terms of pure luck (how the cards fall) which is what I consider as largely constituting variance, then multiple tables don't change anything. However if you subscribe to the notion that the expected decrease in bb/100 winrate equals a direct increase in variance (without considering the respective *increase* in hourly rate), then yes you could say it does increase variance somewhat. But not drastically. Remember that as bb/100 goes down, your hourly goes up if you're a successful multi-tabler. If it doesn't then you're losing money and have to make the same sort of decisions you'd make with a given bankroll while suffering a downswing at one table, i.e. if you're no longer properly rolled for the stakes you're playing, then you need to move down, decrease tables until winrate improves, or boost your BR.
If you're losing money at however many tables you're playing, then BR is irrelevant at that point in time anyway, and you need to move down/decrease tables until you can sustain/increase your BR.
Yes, I completely agree, our bankroll does control our actions, I forgot to put that in my reply sorry, you need a lot bigger bankroll to play 4 tables than you do one.
Not so! Bankroll has no considerable impact on how many tables you play. Whether you play 100 games one after another or 100 all at once, the BR considerations are the same. As I said above, variance is arguably increased with more tables, but not significantly such that you need "a lot bigger bankroll" especially if you're just talking about 4 tables. Maybe if you have 30 BI's and sit down at 30 tables at once, then yeah you're stretched a bit thin since you can't reload at any table during the session. But most multi-tablers aren't (or shouldn't) be doing that.
If you're in the camp that feels variance increases with more tables due to reduced winrate, then a modest adjustment should be all you need. If you are *properly* adding tables, which means maintaining profitability at an increased hourly rate, while also properly rolled for the stakes you're playing, then I argue that there are no real BR considerations to be made.