I think the biggest constraint is freezer space. How many chickens can you fit in your freezer at once? True, you can can poultry, but freezing is by far the most common method to preserve meat. If you have space to raise a hundred birds, and enough money to feed a hundred birds, and the guts to process a hundred birds, but only freezer space for fifteen, you're kinda outta luck. Plus, if this is your first time, esp your first time processing birds, I'd keep it to 25 or less. And even that many birds take up a LOT of freezer space.
I'd say the biggest question relates to processing. If you haven't done this yourself yet and plan to...I'd go with 26 (due to price break) or less. Processing 26 birds for a first timer will be plenty.