Any number of chickens will improve the garden. Do you know how much manure your garden needs each year? Various online source tell me that hens produce about 130 pounds of manure, per hen, per year. Of course they also help compost the other materials available there, and you may be adding bedding too, so the total compost-per-chicken will be more than just their manure each year.
If you know how much nitrogen your garden needs each year, or how much of some other nutrient, you can look up the nutrients in chicken manure and calculate the right amount.
I would probably start with 10 to 12, and see how it goes. But if you really want to start with 50, you certainly can do that. If that turns out to be too many, you could presumably sell some of them as laying hens until you get the number down to what is right.
You might want a large amount of manure the first year to boost fertility, and then smaller amounts in later years to maintain it.
If you were willing to eat meat, or sell meat, I would recommend you try Cornish Cross meat chickens, raised for 8 weeks and then butchered. They will produce a lot of manure fast, and after the first batch you could decide how many batches you need each year to produce the right amount of manure.