The minimum boiler charge is enough to leave your element covered when you finish, use the distillate to water calc (calculator in the newbie section) to work that out. If your on gas, as little as you want
http://www.aussiedistiller.com.au/xcalcs.htmlOtherwise 20% head room is considered a good safe level, I personally run up to 45L when stripping, puke prone washes (eg rum) are better off at the lower level
You need to do enough strips to get enough low wines to do a reasonable sized spirit run, however, the more you have, the longer it takes to run, if you have a still that produces at 2L/h, 20L of 40% will take 4 hours plus boil up time (assuming 95% output from the ccvm), you would need to strip 90-100l of wash to get that 20l of 40%
The larger the boiler charge, the better the cuts though, all the different components are more likely to congregate with themselves in the column giving you better separation in your cut jars
Subsequently, many end up with 200l fermenters as this will net you enough to do a 40l charge of low wines - give or take. If your in vic/sa/nsw, olive drums are readily available, have a good sized opening and are pretty cheap (15-20 in vic) I have managed to get them for 45 up here in QLD, but can't locally atm