The general advice is to keep at least two hens, so that being alone is never an issue. They are social animals, and need the company of others. But at the same time they are independent too.
A handsome Gingernut Ranger - independent minded, but in need of some basic TLC
This means a hen left on her own will just get on with being a hen and do all the things a chicken does. If you have one hen left after the other members of your flock have died, she will not need any special care and attention. If you plan bringing in more hens when point-of-lay birds become available, all well and good.
A single hen who lives with you like a household pet will still be happy spending her nights in the coop, and will not miss having other birds around - as long as the coop is secure, and as long as she has your company to look forward to.
Can Chickens Fend For Themselves?
Yes and no. They can sort themselves out, as long as you provide them with the basics each day.
Hens need a certain amount of looking after. Their food and water need tending daily, and they must be let in at night and let out in the morning.
While an Autodoor can assist in the morning and evening duties, you still need someone on hand if you plan being away from the hens for more than 24 hours. A friend or neighbour may be willing to take on the basic duties - for the price of a few egg!