At this time, there isn't a way to limit the number of contacts that sign up through the form. I have put in a feature request and we will start tracking the interest in having this added.
Timothy_G Community & Social Media Support
Did I answer your question? If so, please mark my post as an "Accepted Solution" by clicking the Accept as Solution button in the bottom right hand corner of this post.
You're not alone on your email marketing journey. Connect with a Marketing Advisor to see how they can help you develop the right strategy for your business and get the most out of your Constant Contact experience.