Should you limit the number of participants in your conference call? That depends on the purpose of the conference call. Perhaps you have never considered putting a limit on the number of participants in a teleconference because it seems like you would want as many participants as possible but then again you may want to consider a participant limit to be more effective.
If the sole purpose of the teleconference is to announce something and give listeners information that you really want them to know (and tell others), then you should allow any and everyone to participate. When you set up a teleconference as a way of publicizing new products and services, you may or may not want to limit the number of participants. On one hand, the more people you expose to your company and products, the better. On the other hand, you have to know that listeners will not feel special or singled-out.
Conference calls that are set up as a dialogue might benefit from a limited number of participants. Publicizing this limit will may make the truly interested sign up quickly. If you want time for the speakers and participants to take an in-depth look at a topic, then a limit will give them the opportunity to do just that. A moderator will still be needed, but that person probably won't have to do as much as they would if you had a large number of people involved.