In the beginning, we start by being aware of an obvious object – one object, two objects – that’s enough. You can use anapana or rising and falling; any object, not all six objects.
When the mind is sharp and fast, then it can naturally know several objects. In the beginning, one by one is enough.
You start by being aware of some object; and slowly when the mind becomes stronger – awareness becomes better and better – it can naturally receive more objects: you can be aware of breathing and thinking at the same time. It comes naturally.