I have self-published two books so far, with a third in the making. I don't think there is a concrete answer for your question, as sales depends on a combination of factors. Some factors to be considered are topic, interest in that topic, pricing, how much marketing you have done and whether or not you have a following before publishing. My books have had very few sales so far, but I do anticipate with time it will get better. I have also read that the more a person writes and publishes, the better the chances of making sales. There are exceptions to the rule and some authors become overnight successes, but for the most part it is a lot of hard work. Do not let this deter you from publishing, but you also have to be realistic in your expectations.