The number of heads you use is the main thing that determines the strength of the flavour you get, i.e. 20 heads will produce a much stronger flavour than 4.
The amount of sugar used is almost as important though, as the initial fermentation will drive off many flavour compounds. This means that for higher ABV wine/champagne you need more flowers to get the same strength of flavour. The amount of sugar used in various recipes for elderflower champagne can vary quite a lot (I've seen it as high at 2.5Lbs/gallon) which means the length and strength of the initial fermentation also varies widely, and therefore the amount of flowers used needs to be adjusted accordingly. A recipe for 4 heads could easily have more elderflower flavour than one for 8 heads, depending on the amount of sugar used.
If you're doing a wine/champagne with an end ABV of 12% or more you do have one alternative which lets you stretch the flowers quite a lot further. Instead of adding them right at the start, you wait until the alcohol level has reached ~10% - usually about 5 days, or a drop of ~075 on the hydrometer. You then use half the amount of flowers you would normally. That way you avoid the destructive initial fermentation, and the higher level of alcohol will draw out the flavour compounds more effectively than the sugar-water you started with.