The number of follow-up emails can vary based on the objective of the campaign and the customer journey. A common approach is to send a series of 3 to 5 emails. The first email should be a gentle reminder or thank you message, followed by more informative or persuasive emails that provide value and address potential objections. It's important not to overwhelm your subscribers with too many emails, as this can lead to unsubscribes or spam complaints.