I think the most I ever had going at once was 14? A few with one person. I had maybe 3 going with one partner.
I don't know how I managed. I have no life, and I'm very quick to read, imagine, and write my responses. I can spit out several paragraphs in ten minutes. 
Of course they're riddled with errors, thank to my dyslexia and my complete lack of observation, but eh  xD

*hugs coffee cup*
its rainy and gloomy in Okinawa. I just wanted to sleep.