You’re welcome! 
Only you know what you can handle, it’s really not for anyone else to tell you what is right or wrong. Everyone is different.
For me I can handle my 10 stories perfectly fine and still manage to maintain high quality writing and directing every single time. [Is it tiring and stressful?! Ugh, like you wouldn’t believe… but the end result is so worth it.]
Focusing on just one story is actually dangerous for me, because personally… that’d only end up resulting in writers block. And I couldn’t go all that time without writing something. Writing has been a huge part of my life and such a huge passion of mine, ever since I was a little girl.
But as I said before… my technique may not work for you or others, just as theirs certainly wouldn’t work for me.