1) For instance cleaning or even on feeding day.
I have a variety of species, all on different schedules. It gets complicated, but I plan out when to offer food, clean cages as needed, and write it all down on a google spreadsheet document.
2) I keep them on a certain feeding schedule and sometimes half of them eat, or one. it varies.
Again, I just use my document. If it's been long enough since the last successful feeding, the snake gets offered food. If they refuse, then I wait until the next scheduled feeding day.
3) I feed live so I like to watch to make sure nothing bad happens (lol to the snake)
Feeding F/T is paramount to my sanity. I may get more refusals with one or two individuals in my collection by feeding F/T, but every animal in my collection takes F/T if they get hungry enough. Really the only big pain in the butt for feeding is our male rainbow boa, so he gets special attention to try to get him to take F/T more regularly.
4) Also I try to handle all of my snakes several times a week to socialize them. What do you do when you have 20-30-40 snakes?
We handle all of our snakes. Some only get maybe 5 minutes of handling a week, or less depending on their mood (see nippy young carpet python or blood python stereotypes that sometimes turn out true). Others get handled more often. I can guarantee you though that businesses with hundreds if not thousands of snakes don't handle many of their animals much more than picking them up to clean tubs. In addition, plenty of breeders don't handle animals who are breeding, so as to not add any undue stress.