The rarest resource in The Walking Dead: Survivors is bullets and although there is no limit for bullets, there is a limit to the amount of bullets you can take for a survivor regardless of how many bullets you have in your inventory. The limit of bullets you have available for a formation increases as your power increases for the formation.
Swap your survivors around your formation as the stamina finishes. keep rotating all your formations to maximize your bullet resources.
Gunshops and Army Camps
Assign any spare survivors you have to Gunshops and Army Camps. Increase your Gunshop levels to increase bullet production. You will be able to unlock and assign survivors to an Army Camps once you have beaten a level 19 walker. Army Camps are not easy to find on the map.
Find a Herd of Dead Accounts
Dead accounts that haven't been usied for a long time will have been stacking up bullets. Finding a herd of them is a good place to farm bullets. Just make sure you don't 'zero' (make the bullet count drop to zero) them so the account disappears. You can identify a dead account as there will be no troops there.
Events and Survival Challenges
Keep an eye out for the Tides of the Dead Event and survival challenges and participate in them whenever they become available as these are two ways in which you can get a decent amount of bullets.
Visit the Lone Trader
The Lone trader is usually opens before the survival challenge begins and it allows yo to buy or trade other resources for bullets. If you are low on bullets it is essential that you visit the Lone trader to stock up.
Certain survivors such as Zoe and Eugene give bullets every 12 hours and can also increase bullet production. Also, Sarah's skill can reduce bullet cost when making a building when assigned in a worker's cabin.
We have questions and answers related to this topic which may also help you: