It can sometimes be confusing to figure out how many subscribers you need to sell a specified number of artworks. Until you have enough hard data on your experiences, using the direct marketing standard percentages will suffice.
A direct marketing campaign that achieves a 4 - 5% closing rate is considered successful. If you are able to get higher percentages you are doing many things right. If you are getting much lower percentages, you need to review your marketing. It is important to learn where in your customer journey (sometimes called a funnel) where your marketing processes are inefficient.
Use this calculator to determine approximately how many interested subscribers you need to sell your desired number of pieces annually. The results are a great starting point. You can build your list bigger and tweak your marketing to improve your results.