How many in one enclosure? One, and only one.
Many, many things can go wrong when housing multiple animals in a single enclosure. They're not like rodents who thrive on company and companionship, they are a solitary creature, and should be kept as such.
Stress is a big unknown with reptiles. It can result in a weakened immune system and what would normally be a minor issue can kill a snake. When you take a solitary creature and house it with other snakes, you might end up with snakes who won't eat or end up dying from common problems.
One of the new snakes may have some sort of disease or parasite, and what was only one sick snake is now 6 others. Herp vet bills aren't cheap, and when you multiply that by 6 it's expensive as all heck. If one of them has a problem, you don't know which one has the problem. They don't label their poo's or regurges.
Cannibalism. True it's rare among corn snakes, but why take the risk?
For me the choice is simple, separate enclosures for separate snakes. I choose to keep my snakes as healthy and stress-free as possible. Take the time to buy enclosures for each of your snakes. It doesn't have to be an elaborate setup, which can be expensive. Lots of people on here do things in unconventional and thrifty manners.
But take it from experience, cutting corners in reptile husbandry can quickly lead to disaster. Research this forum, find ways to make enclosures for your snakes that won't break your wallet.
Best of luck to you, and your snakes.