I remember going through 16 interviews to be hired by a top tier Wall Street investment bank. I was an experienced hire, lateraling over at this point
I was only hired after I got an offer from a competitor (following about ~6 interviews there, most of which were over the phone) and forced my top choice's hand
Fast forward 18 months and I was laid off along with hundreds more. As it turns out, my two superiors weren't liked at all, so when the market slowed, their peers took the opportunity to get rid of them... and I became irrelevant as I had nobody to work for.
In hindsight, it's somewhat obvious that it took them this many rounds because they were not certain they needed someone to work for the guys everyone disliked. I got a couple of deals done, had a couple more lined up, generated millions of dollars of revenue to the bank, clients liked me... but it wasn't the right place or the right time.
If I could do it all over again, I would have been more upfront and a tougher negotiator with them from the outset. I either want full commitment or it's not worth it. Tell me upfront how many rounds of interviews you need and I'll tell you if it's worth my time and the brain damage.