The number of stakeholders acting is the count of distinct individuals who have completed one of your actions. If a single individual clicks a link to take action twice (using the same information), they will be counted only once as acting (although the two actions taken will be captured in their stakeholder profile).
However, if a stakeholder were to have two profiles (say, one with professional contact information and one with personal information) and they took an action with BOTH sets of information, they would be counted as acting twice. Note: you can merge duplicate profiles for advocates easily.
Number of acting stakeholders is based on individual profiles within the database.