Does app review depend on app reviewers availability? And holidays etc?
Yes, of course. If there are no reviewers available, then there is no one to review your app.
And yes, Apple does regularly shut down app reviews for significant holidays.
IS there a maximum number of days that an app has to be reviewed before end of that period? In other words: if an app is posted, a review HAS TO HAPPEN before x days, is a rule (hopefully) or no?
Nobody outside of Apple knows Apple's internal app review procedures. However, we can make logical inferences.
Let's suppose there is a rule that a review HAS TO HAPPEN before 10 days. OK, then. Now let's suppose that doesn't happen. We can suppose that because obvious you wouldn't be asking otherwise. 😄
Does your app get automatic approval? Obviously not. Is the app reviewer who didn't make quota subject to some kind of internal discipline, and possible reassignment, or even termination? Sure. That's possible. Then your app would be reassigned to a new reviewer, who hopefully gets the review done within 10 days. But there is no guarantee of that. Apple could burn through slow reviewers forever and your app would never get reviewed. Therefore, if there is such a rule, it applies only to the performance evaluations of the app review team and is irrelevant to when, or if, your app ever gets reviewed.
Once an an app has been reviewed once, and you send a new build (that addresses the fixes) for example you add the "sign in with Apple" if you forgot to add it next to "sign in with another method", does the second round of review have to be done by the SAME REVIEWER first time or can it be done by a second reviewer?
Again, let's use logic. Suppose there is a rule that the SAME REVIEWER must review all subsequent submissions. Furthermore, let's suppose this reviewer gets hit by a bus. Too bad for you, I guess. Or what about scenarios that aren't as sad. How about a wedding, or a honeymoon, or a vacation? Do you really want your app to be sitting in a reviewer's inbox while the reviewer is downing margaritas on the beach for two weeks?
Therefore, we can logically assume that there is no such rule.
I have posted and have been waiting since 5 days now (since 5 march)
Over a weekend? I should point out the existence of a concept called "business days". One of the reasons someone might want to work for Apple is so they don't have to work weekends.
Are we allowed to post many apps at once?
Sure. Why not?
I mean as a developer I can take time working in multiple apps, then suddenly deciding to publish them all, that is okay right?
Sure. Go for it!
Are their limitation or expected "published flow" to have?
Well if you were the world #1 target for scams, would you implement some kind of system to detect suspicious activity? Isn't it reasonable to expect some a rate of new app releases and/or updates that are proportional to the number of developers on a given team?
Once an app has been approved and published, will the waiting times for updates (new versions of the app) reviews smaller hopefully?
Hopefully. 😄
or how was your experience? Is there a general rule you have seen?
One developer's experience is not applicable to any other developer's experience. Maybe I develop some dictionary app for medieval French literature. My experiences may not be applicable to your innovative new social networking-crypto-dating app for kids.