First it depends on what kind of eBook and price you are offering. Without knowing that I am going to assume that you have set a price of $20 for the book. The reason is because anything lower than $20 then I would not bother to take the time to promote it.
1. Get a seller's account in jvzoo.com and clickbank.com
2. Buy a cheap domain name (prob .info) and set up a mini-site with that domain name.
3. Then I would write a high quality content article on the book and submit to ezinearticles.com. While that was waiting to be approved manually through them, I would spin the article I have just written and submit it to goarticles.com ensuring that I had my new domain address where i have the book stored at in the author's box.
4. Jump to Facebook and get the word out on it and also with Twitter.
5. Finally I would go to Amazon.com and list my new ebook with them.
As for a realistic number of sales, that is not easy to equate because there are many factors that are involved. If you are not already credited like say Stuart Woods or Stephen King, then shooting to be on the best seller list is out of the question. It also depends on the niche that your book is in. I am pretty sure that weight loss, internet marketing, dogs, etc. niches will sell way better than underwater basketweaving.
Something to remember is that your ebook is digital so do not focus on the sales aspects but rather focus on something a little bit more tangible in the business world. Your reputation, if you build a list of followers then you will find that your book(s) will sell like hotcakes from your devoted and loyal followers.
Hopefully this was helpful for you.