I did a 10k, then 5 halfves before my full. I had been running for a solid year and a 3 months. I thought I was ready mentally for my first Full but I wasnt. the last 10k was he-ll.
I dont think it is how many halves you have completed. Yes, those give you experience in pacing and not getting caught up in the pack and running against you "plan", but it is the stamina and mental stamina that I gained over the next year of just solid running (not necessarily in events) that led to my sub 4 negative split full.
It was experience, not events that did it.
EDIT: Yes, events give you experience from a hydration standpoint, nutrition restrooms etc, but if you do the full schedule, you will have to address these issues too which will make you more independent and not rely on the events location of water stations, nutrition stations etc.