How many projects do you work on at a time?

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With Darkness Within released, I move forward to the next book in the series, Light Without. However, I feel like I have opened some sort of creative floodgate and ideas for new projects just keep flooding out of my brain. I have at least 5 books in my immediate thoughts and when I set down to write Light Without, these other projects jump in my face for attention.

So, I was wondering how many projects I should work on at a time. I know I can write one book while outlining another but could I write 2 books while outlining 3 more? I’m sure this is a personal preference but I thought it would be a good idea to get an idea from other indie authors on the matter. How many projects do you work on at a time?