I too am applying to the Peds NP track this year. I applied last year and was wait-listed after my interview, so I'm REEEALLY hoping to make it all the way through this time.
As for the numbers, I think there were around 600 applicants last year and around 120 were invited to interview. 60 were offered admission after interviewing. The number of people they take slightly varies each year (I think it depends on funding), but if you make it to the interview stage you've got a 50% chance after that. FNP and midwifery seem to always be the 2 most popular, and therefore competitive, specialties.
Also, not to scare anyone off but I remember last year they cut one of the programs mid-application cycle. So some individuals had applied for a masters specialty, and that specialty got cut after the deadline. Unfortunately, those people were just out of luck as UCSF did not allow them to submit a new application to a different specialty. I got the sense that they felt very strongly that pursuing a program that offered your chosen specialty should be more important than just wanting to get into a certain school (e.g. UCSF). Based on that, I would recommend that you be very clear in why you want your particular specialty if you do get chosen for an interview.
I received notification of being selected for an interview the first week of December last year. I just checked the letter (yes, unfortunately it is snail mail) and it was dated December 2nd, but I think I recall actually receiving it around December 7th or 8th. Last year, I was told by an admissions staff member not to expect it until mid-December, so I was surprised to receive it earlier. I think in general UCSF gives very conservative notification expectations so they don't get bombarded with emails and phone calls if they end up being late.
Hope some of that info helps and good luck to everyone!!