The great thing about Campden tablets (a convenient form of dosing in sulfur dioxide for home winemakers) is that they will inhibit the yeast and bacteria you do not want (which are sensitive to sulfur dioxide) while allowing the yeast you do want to continue to power through the fermentation. The little packet of yeast included in a wine kit, or pouch that you purchase for your elderberry wine recipe, contain yeast species and sub-species specially grown and formulated to tolerate a little bit of sulfur dioxide. They’ll be able to power through a fermentation safely with a little sulfur dioxide while “bad bugs” and their “bad behavior” will be repressed.
One crushed tablet in one gallon (4 L) of wine produces about 65 ppm (or mg/L) of total sulfur dioxide. Since I’m not sure which recipe you are basing your wine on I can’t quite tell you if one tablet is too much or not. Just be sure you don’t use more sulfur dioxide or Campden tablets than a recipe calls for. Measure twice, cut once and ferment on!