It's like 20-30. I've gone through about 18 fills on a 400g canister and there's still a lot left.
I wouldn't get a loader, as they can feed BBs pretty fast and they won't settle properly in the magazine if you fill it too fast. It's best to do it a couple at a time, to make sure you fill it up right. If you just want speed, then get a BB bottle with a little spout and they'll pour right in.
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