Thank you for reaching out to the Community about your contact list! I can understand how helpful it would be to place this limit on your sign-up form. However this is not a feature we currently provide. At this time there is not a way for the sign-up form to automatically close once your list has hit a certain number of sign-ups. Instead we suggest manually removing your form. I apologize for any inconvenience this may cause and have submitted this feature request in your account in the meantime.
Frankie P. Community & Social Media Support
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