Three, four, and so on. This rule has been the key for me compounding my skills and growth as an artist over the years. I started this practice circa 2005-7 when I was getting serious (child terms- having fun) with photoshop -- before CS! Here is how it worked.
Let's say I was working on a desktop wallpaper and I was learning how to use new techniques than normal. Whether I love the work or not, I do a second version immediately after. This doesn't mean I don't sleep. I would work on the second wallpaper as soon as possible to tweak and test the limits that I may not have come across in the last one. I have found 8 out of 10 times, the second work is much more successful overall.
I felt I took more care. I still remember what I loved and what I couldn't get to work. Sometimes I just can't find the right colors. Sometimes it is just a poor subject or reference I used. But I always end up learning about composition, balance, tonality, and every other aspect of art because I immersed myself into creating. In your mind its all mixing together like a beautiful stew.
I understand that every medium has its hang ups and differences but this is important if the goal is to "speed up" your learning process. If you make movies and you could only do one at a time, how can you spur up the next idea before the first one is finished? Painters have different dry times for their paints. Some like to focus on one a month. Some paint 6 at a time, also revisiting unfinished paintings to revamp. As long as the second artist is working with the same care and diligence, they will improve their technical ability much faster than the first painter. If your ambitions are high and you expect to show in the Guggenheim, you better paint and sculpt until your hands go numb. Now go create!