I have three stories on sub right now, which is the most I've ever had out at one time. In fact, these three stories represent the sum total of my short fiction inventory. My efforts to become more prolific have been... about as successful as my efforts to get a driver's license.
I gave up trying to get a driver's license. I won't give up trying to be prolific; it's too much fun, and also, if I screw it up, no pedestrians' lives will be at risk.
I suppose I hope that while I'm low on the quantity axis, I'm proportionately high on the quality one. I have a low sample size with which to judge, but for what it's worth, my stats are pretty good (all sales pro so far; few rejections per story; when rejected, always personal and inviting me to submit again). This is, of course, whistling in the dark... I haven't made a sale in a year, and I'm frittering away my momentum, especially if none of my three hopefuls make the cut.
They are lovely little things, in my opinion. And I must make more of them.