as for 3, I think you can milk each animal once a month. so probably 6 or 7...
it takes 20000 steps for the milk to be recharged on each animal. but 6-7 should be pretty close to how many are necessary for a milker of some skill. As they progress in proficiency, you will need to acquire more dairy stock. (Make sure your dairy stock has desirable attributes such as "enormous" "muscular" and "fat." This will help maximize the amount of material recoverable from each animal. Enormous affects bone mass, muscular affects meat, and fat, well, duh. I shoot for enormous and muscular but dont care much for fat. If you are producing much soap then that could be useful.) I would like to see tests done on this matter...
Another consideration is how efficiently iis your workshop area designed. Meaning, are the chains adjacent to the workshop? Is there an empty bucket stockpile nearby? Is there a liquid animal extract accepting food stockpile nearby? Are you sticking with one dwarf milking or will multiple dwarves have the labor enabled (to allow for truly continuous milking)? Will there be multiple milking stations?
(protip: make sure you have a second liquid stockpile away from the milking plant that will take from the near stockpile. This will let you manage your haulers such that milk will be brought to the larger stockpile and then transfered to barrels (possibly have 2 stockpiles for this.) so that when processed into cheese, you get stacks of 5 and not single stacks of cheese.)