How to tell the difference between a Clicker and a Runner in Listen Mode

Depending on the difficulty, you'll find more or less items during your quest for survival. The harder the difficulty, the less items you'll find. You'll still be able to find all collectibles regardless the difficulty you're playing on.

Not all items disappear on harder difficulties. Weapons, Skill Pills, Parts, Training Manuals, Comics and Artefacts can be gotten during any of your playthroughs.

The harder the difficulty, the more damage you take from enemy blows. Some enemy attacks retain their instant-kill effect on all difficulties. Enemies will also notice you considerably more quickly the harder the difficulty.

The Easy difficulty also has an additional option for 'Lock On Aiming', which makes it easier to shoot targets.

The Survivor difficulty doesn't have a Listen Mode nor Melee Prompts for in-game combat.

Source: Walkthrough by Absolute Steve

Just use Last of Us Wiki to get info of maximum bullet capacity of a weapons. And according this forum and my personal memory, each magazin upgrade adds one more bullet to maximum capacity (or maybe a little bit more, maximum bullets of unupgraded weapon plus bullets in magazine).