For oil derricks it takes 26 to capture a neutral and 52 to capture an owned one. If it appears to be taking less than that, it's just client/server desync (i.e. you've actually used more charges than that, but it doesn't show, and the extra charges you appear to have saved don't work because AOE damage is server-side). So at the very most, you can capture 3 neutral derricks, capture 1 neutral and 1 owned, or capture 1 owned and turn another owned neutral, with the remaining 22 charges being leeway to make it less likely that you'll end up using too many charges to capture those specific amounts of derricks.