Sometimes it's wise to work on multiple pieces of art at the same time. This practice can help you manage time that will be taken at the later stages of a painting. It can help you create a very cohesive series of paintings as well. Here's a few tips to get you started.
1. Plan which sizes you are going to work on and buy all the canvases at once. Over estimate the amount that you might need by at least one or two. This way you have room to abandon a painting that isn't working or you have extra work to sell or exhibit.
2. Prep all the surfaces at the same time. I usually include a few base coats of colour applied really loosley at this stage so that I can let texture show through. If you are working on enough surfaces you should have one almost dry by the time the others are painted so that there's no idle time between coats.
3. Write a list of all of the ideas that you might include in this series. It can be a list of objects, subjects, types of colour combinations or forms. Keep the list on the studio wall so you have all your ideas handy.
4. Dedicate particular canvases to particular ideas. If some of your canvases are small and some large - decide which ideas lend themselves better which shapes and sizes.
5. Plan it out on the calender. How many per month? Break down the work into weeks and then days. Everyday that you are in the studio - check your progress so you can stay late if you need to catch up or adjust your plan on your calendar so that it's a little more realistic.
Good luck on your new series of paintings!