You get a couple of turns and then it’s game over due to no ammo. The stuff you find in crates is never what you want. What do they carry into battle, half a clip?
You normally start with 1 mag loaded in your weapon and a spare mag in your pack. If you find ammo on the map. You can also equip different weapons that you find on the map if you don’t have the ammo you need.
You can take additional ammo with you on missions from the equip screen and you can manufacture ammo for any weapon you own at a Phoenix base for a few material resources. It is all in the game guide provided with the Backer Build.
That’s strange, I almost never run out of ammo during battles, and I never open crates during tactical battles. Maybe there’s an overwatch/return fire abuse, or not preparing well enough before battle, not packing enough ammo? Don’t forget you can manufacture ammuition to all your weapons in the inventory screen
Maybe my tactics. I should be heading to, and securing crates instead of fully concentrating on the enemy.
First of all, players should focus on staying in cover, second, they should focus on shots that hit enemy weaker body parts, thus getting in right distance where shots are accurate enough. 
Crates are not important. You can get to them when you finish all that is between you and crate.
This is until the aliens start attacking crates, which will happen in the next build. We will also put more useful stuff in them.
It’s usually pistol ammo in the crates. 
I find myself giving face huggers priority as they are the most dangerous due to my sentry gun killing who ever it’s attached to, plus other nasty face hugger events.
Oh damn. Those crabs are too intelligent.
Great news.
You have bad luck then. Of course if you not mean crates in havens - those have almost nothing in them.
Maybe you’re wasting ammo in trying to get almost impossible shots? I know back in BB1 I thought my sniper would be like FiraXCom, an unstoppable death machine, so I would waste all ammunition in too far away enemies. And that was in BB1, when alien mutations included super x-ray telescopic vision that would always hit their return fire
Well, there goes my strategy right out of the window! Guess those nasty bugs do evolve and get smarter 