3rd Retainer by Default, please?

[Intaki]

I never said you needed to farm ventures. The point I was making is that the player can chose to do something that earns ventures in order to fuel their retainers actions, or they can chose to gather/farm whatever items themselves, or craft, or do something else entirely.

There is literally no activity in this game outside of plain old crafting that does not carry the potential to reward you with ventures. Leves reward ventures, hunts reward seals which reward ventures, FATEs reward seals which reward ventures, dungeons/raids reward gear which reward seals which reward ventures. If you're not swimming in either Allied/GC seals or ventures themselves, you're not playing the game.

You say that's all game play, fine it's all game play, but does me having 4 retainers mean I can sell things for twice the price of someone else?

No, you just acquire them at twice the speed.