When you create a Scheduled Broadcast and target it only to an audience tagged with a specific Tag, a question may arise: how many people will receive this Broadcast? The answer is: everyone who's got this Tag by the time the Broadcast is sent will receive the messages.
The list of people who will receive a Broadcast is formed at the start of Broadcast delivery.
For example: When creating a Broadcast you have 200 subscribed contacts with the "sale" Tag. The Broadcast is Scheduled for the next Monday. When Monday comes, you have 2500 subscribed contacts tagged "Sale", so the broadcast will be sent to all 2500 of them.