Internship to full time and Exit interview

I'm doing an Audit Interview this summer at one of the Big 4 Accounting Firms in Los Angeles. I know you do an exit itnerview, where you're told whether you got the offer or not. I was wondering when that happens. Is it on the very last day of the itnernship?

Also, since we're talking about full-time offers, does anyone know of specific cases when people didn't get an offer n why that was?

And, last but not least, is there usually one intern per engagement? Or will they put multiple interns on one engagement if they have multiple seniors?

Thanks for any input!!