How do I build more farms?

Farms inside your base offers steady food income but not too much. Even with Heroes and researches it will never be plenty.

You can get food from the map:

If you got stamina (restores by time) you can hunt zombies, you will receive all kind of stuffs including food cans.lvl1-9 zombie hunt's costs same 5 stamina, lvl10+ 10 stamina so its usually best to hunt the highest possible you can beat. When lvl9 are weak you can switch to lvl10+ but they cost 10 stamina.

Also you can hunt lairs, but for those you need 2 players and stronger troops. Always scout if not sure how many are there. (scout info detail depends on your watchtower lvl)

There are foreign farms on the map, you can send your troops there to gather. No stamina is used. Beware gathering troops may be attacked by other players. So my advice send a little more then needed and all 4 kinds of troop, not just vehicles for example.

Doing the quest things (campaign, etc. like build a lvl5 farm or complete chapter 5 etc.) also rewards food.

Since your base can be plundered if beaten best to leave cans unopened, only have food amount that is protected by warehouse. When low open more cans.

If you got spare food you can give it to allies and also if very low ask for it from other players. But its easy to just gather and hunt.