Snakes don't need to be watered every day, they don't poop or eat every day, and they definitely don't need to be cleaned up every day. I clean my snakes once a week, and feed the following day it takes me 20 minutes to clean 15 snakes which means it takes me roughly 1 minute and 15 seconds to clean 1 cage.
Let's do some math shall we?
1 Minute and 15 seconds broken into normal 1-100 numbers is roughly 1.333~, times that by let's say 1000 snakes you roughly get:
1333 Minutes, which is roughly 22.2 Hours which is only a little less than 3 days worth of work. That is IF you clean every single cage once a week, which isn't always necessary.
This doesn't take into consideration what methods they may have for cleaning or what kind of materials they used. See I use ceramique bowls, and paper towels so if the snake has made a mess it takes me a little longer to clean their bowl or to clean off a urate. But if you have 1000+ snakes this is a problem you've already resolved by either spot cleaning or rotating tubs, or maybe even both, and using disposable water bowls.
Now snakes don't need to be fed every week, feeding a snake bi-weekly is just fine, most people advocate feeding every week because a lot of people have intentions to breed which if that is the case weekly feeding is a wise decision. But when you're selling snakes, you're likely feeding them bi-weekly, which is still more than enough to grow a snake and to keep it perfectly healthy and happy.
So really keeping up with that many snakes isn't as difficult as it may sound, Barcyzk keeps up with 10x that in his facility with roughly 5-6 employees.