I'd recommend as many as possible. DS started at 9mo (settling in at 8mo) and I did a couple of weeks every day, building up time away. Then I got my mum to stay for the next two weeks and do early pick ups so it was well over a month before DS was doing full days.
The nursery didn't charge for the settling in period. I didn't stay for full days - just a couple of hours - because obviously that would have been very disruptive. It was more about going every day so DS got used to his new routine and leaving him there for longer and longer periods.
I followed the nursery's guidance (leave quickly, don't dally) but I felt my DS needed a big run up to full days as he was very clingy/still breastfed/bottle refusing. The staff told me each child is different - some kids are happy staying alone there after one session, whereas others take months to truly settle.
I'd aim for as many settling-in days as you can arrange. We had 5 - he was due to start on a Wed so did Wed-Fri and then Mon-Tues. the manager pointed out that he'd forget over the weekend. However we've found the first week was not full days so it was lucky we were able to collect early...that's as far as we've got so I can't say how the 2nd week went yet! I'll be working from home though, just in case...