It seems like a major oversight that this isn't included in forms. The one/two-step order form is no good because it doesn't let us use custom fields. It seems like a small step to be able to add this feature to forms as well - that lets us limit the number of times a form can be filled (i.e. for a free webinar with only 100 seats) - there is no other method in GHL to limit a form like this.