A couple of factors must be taken in consideration.

The size of the land
If it is grassland you need a much bigger area. If the land is densely bushed you can hunt in a smaller area.

The minimum size for a densly bushed farm is at least 500 ha. For a grassland farm at least 1500 ha.

Animals on the farm.
Questions one must ask is:
Do the rancher buy in stock for every hunter that vists his farm. Put and take is a NO NO.
Are the animals that is hunted on the farm, born and raised on the farm. If yes you can hunt on that farm, the animals will know that farm inside out and you can prepare for hard hunting.

If a farmer buys in animals, when does he do it, the beginning of the hunting season or at the end of the hunting season. If it is at the end of the hunting season and the animals are left to learn the farm for at least 4-6 months it will be OK.

Hunting Methods
If you are hunting at water holes, artificial feeding places it is a NO NO. It is totally different if you are hunting next to a river, it is not an artificial drinking place.

Any area on a farm that is subdevided in small high fenced camps to contain a certain species is a NO NO

One of my clients wanted to hunt a Njala. I spoke to a farmer that had Njalas on his farm, on questioning him about the animals he told me there were 3 matured bulls in a camp of a 100 ha. I advised my client that it is a NO NO, it was shooting and not hunting.

Yes there are certain animals that you will not get unless you bait them, Leopard, Lion and even Bush pigs are difficult to get without bait. I am not all for it, but it is the clients choice.

One can also reason that driving around in the bush to find animals and then get off the vehicle to hunt them is a canned hunt.

A lot of the cotroversy depends on your own idea of what canned hunting is and what is ethical for yourself. In different areas in the world there are different ethics. The end of the day it is the hunter that must decide if it is ethical or canned.