Ammo, Weight, & Rations Tracking

Rules As Written

Ammunition is bought and tracked like any other item.
There is an optional encumbrance rule.
Creatures that need to eat consume rations as described in the PHB.

House Rule

Ammo

Magical and non-magical ammunition should be tracked by the player. Retrieving arrows and ammo is assumed, but buying arrows and ammo is not.

Rations

Rations are not tracked. It is assumed your character is competent enough to buy lunch for the most part.

Encumbrance

You have the amount of slots that equals your strength score. If a character can comfortably carry something in one hand, it's one slot. Anything bigger is two slots. Heavy armour is 3 slots.

Exceptions

If the party is put in a situation where scarcity is a factor and the lack of ammunition or food is a part of the challenge (and therefore enhances gameplay), this rule may be suspended. You will be told if this is happening.
In some situations, the weight of an object is part of the puzzle, or the player may want to take something that couldn't reasonably be carried. You cannot simply put the statue in your pocket because "I can hold it in one hand for 5 seconds". If you want to take out the solid gold statue the size of an ogre, you're going to need to justify carrying it out. If you disagree on whether your character could reasonably carry out that solid gold ogre, encumbrance rules in the PHB can be used to adjudicate that dispute.
As always, DM has final say.