Wellspring will divide its tooltip healing amount by the targets hit, in order to do a maximum amount of 6 * (tooltip healing). This means that, usually, the maximum amount of healing you can get from a Wellspring cast caps at 6 injured targets. The trick to go past this hard cap is understanding it does this calculation as you finish casting it, but it still has a small travel time to actually hit other players. If only a few people are injured at the time Wellspring finishes casting, but they all become injured by the time Wellspring actually hits, all of them are hit by the uncapped amount, thus creating a huge burst of healing! Just be careful not to finish casting Wellspring when no one is injured because, at that point, its unique over-healing prevention mechanic will kick in and it will do extremely low healing on anyone hit, instead.