Hi Andrew and welcome to the forum
You can expect to sell anywhere from 0-100, that's about as close as I could get for you I'm afraid. So many factors in play here that will determine sales. For example, are the prices competitive, are listings going to inspire customers to buy, is the market looking for the types of items you are selling and so on.
There is no such thing as a simple average that fits all, it all comes down to a multiple of factors.