How many people should be on my list to have a successful webinar?

There are too many variables to give a solid answer to your question. How much do you plan to charge? What are your costs (e.g., webinar fees, merchant fees, etc.)? And what will your offer be?

Here is a different perspective for you to consider:

Don't worry about how many people sign up for and attend the webinar. Just do it. If you make money, great. If you don't, no problem.

Record the webinar, and offer the recording as a product. Offer it as a download, put it in the member area of a site, put it on DVD and sell it. Whatever.

I did a free webinar that was designed to survey my list members regarding what they want to learn about creating and selling information products. Everyone who signed up had to submit a question, and I am using those questions to determine the topics of future webinars and other products.

I am now selling the webinar on DVD for $39 from my site and at Amazon.com. And, yes, it is selling: Creating and Selling Information Products

Don't wait until you have a list of x number of people before you start creating products. Do it now, and continue to build your list. Then sell the products you already created to the list.

Your list should contain at least 2000 people, of course only a portion will actually go to the webinar. So if your list is 2000 and they like you and your products and respect your opinion you will probably get 200 at the webinar for a list of 2000.