Re: regarding phone book capacity
The capacity for any information (Calendar, Contacts, Photos, etc) is limited only by the storage capacity of the device. A 16gb device can hold up to 16gb of "stuff" - iOS, apps, app data, contacts, calendar entries, photos, music, videos, etc... None of the specific categories are limited in capacity beyond the overall capacity of the device.
Originally Posted by iMore Question
To see what is there, go to Settings > General > About and see number of photos, contacts, calendars, songs, videos, etc.