This boils down to the definition of a farm. When I read the title, I was expecting to see a list of about 10+ accounts, which is my definition of a farm. So, the question is, what number of accounts belonging to one person can be regarded as a farm?
For privacy reasons, having one alternative account is enough. But if you provide some kind of services like LoyceV and TryNinja do, you might need to create several more accounts for different purposes. I think you don't need 10+ accounts to call it an alt farm.
If someone has more than two accounts, I think that should be considered as an alt farm. As for two accounts, that is just an alt account, and I don't think this could be defined as a farm.