This is a very good question, I'll try my best to help answer it for you
Overall, the answer to your questions varies widely, from facility to facility, and species to species.
For the past three places I've been at, things were as such:
A smaller facility, collection was mainly fish and reptiles, along with some birds and marine mammals. Over my time there, we varied between 16 and 21 animal care staff, and 2-4 education staff. As a general rule, having more staff did enable us to put more back into the animals (in terms of training and handling sessions), however the facility has had as few as 11-12 AC staff for extended periods of times and they still managed fine. We would usually divide staff by four sections, fish and reptiles, birds, flex and marine mammals. Generally 2 a day on birds, 2-3 on MM, 5-6 on F&R, and 2-3 on "flex". Flex staff in theory could move between all three sections in the day, but in practice were usually the newer staff who where either training or tasked with the more "mundane" duties. While I was there we had around 6-7 people who did the majority of the MM stuff, 8-10 who did bird stuff (all MM people also did bird stuff) and almost all the staff were trained on reptile stuff. Keepers would only stay overnight for specific reasons, which usually worked out to around a week or two worth of night shifts a year. The facility was armed and locked up at night, we had security that would do rounds around the outside of the facility ever hour or so at night. In terms of diet prep is was largely fish and reptile staff, but all staff were trained on food prep. Staff were at the facility between 7:30 and 6:00
Another is a medium-large facility, mixed collection roughly half mammals, 1/4 birds, and 1/4 herps. Year round animal care staff is generally in the low 20's, with 3-5 education staff. During the on-season there is a number of seasonal's hired on, generally around another 8-10 animal care hires and 10-15 education hires. For specifically animal care, there are usually around 15 or so people on a day. There are a number of shifts per day. On average its 4 on elephants, 2 on carnivores, 1 on birds of prey, 2-3 on marine mammals, 1 on hoofstock, and the remaining seven or so working with the other birds, smaller mammals, reptiles, and commissary. There are also "unassigned" positions, these keepers will work wherever keepers are needed (often times this shift is very useful for getting exhibit work done). During the on season add another 3-4 a day working with the program and contact animals. Many of the staff are cross trained in different areas, some areas being more "exclusive" than others, and generally staff will work between 2 and three different areas in a week. Keeper staff are generally at the zoo between 7 am and 10 pm. The bulk of the heavy work (so feeding and cleaning) is done in the mornings, then after 1:00ish most of the work is training and more specialized tasks. Daytime staff start heading out between 3 and 5, and are replaced by a smaller amount of night keepers, who stay until 8-10 pm. These keepers do mainly food prep for all the animals in the evening, as well as feeding out PM diets and meds, and sometimes some training and cleaning, as well as patrolling the grounds. After 10 or so the night keepers are replaced by a security team that patrols the grounds until keepers arrive the next morning.
At yet another facility (this one I would classify as large), it by far had the most keepers and ed staff. I can't remember exact numbers, but there were generally 20+ keepers on a day, and several ed staff. There were a bunch of different areas, south american birds, oceanic birds, african birds, elephants, carnivores, african...mostly hoofstock, african primates, asian primates, reptiles, childrens zoo, commissary, quarantine/vet holding and a few other areas. Most smaller animal areas had between 2 and 4, and larger animal areas had between 3 and 5. Commissary was I think two staff a day. Generally, there was little cross training, whatever section you worked in was "your" section. There were some swing keepers who would move between different areas, but generally it was in the different bird areas or whatnot (so no staff going from elephants to childrens zoo, for example). Keepers were there for similar times to the above facility, and yet again with a security team there overnight.
As a general rule, a well funded facility of any reasonable size will have AT LEAST 10 staff on a day, while one that operates with less of a financial buffer will usually have 2-8 paid staff a day, and a larger amount of volunteers, interns, and other non-paid "staff". Often times these facilities are "smaller" (in either physical size or collection size) as well.
