Anyone looking into a retirement savings plan, or trying to pay down a mortgage knows the 'power' of compound interest. Compounding in general applies to stock photography to some degree.
If you maintain some growth in your uploading, no matter how small, you will benefit greatly and grow your sales much faster. Consider two brothers (all financial examples seem to have two siblings to compare!) - one uploads 20 images a month for 3 years. The 2nd uploads 10 a month at first, but promises himself to increase the uploading by 10% a month. At first the 2nd brother lags behind, uploading 10, then 11, then 12 images in the first three months, while the first brother uploads 60. But after 14 months they are dead even. After three years the first brother is adding 20 photos still, an overall portfolio increase of just 2.7%, while the 2nd brother is still adding about 10% to the total.
If you consider that the sales generated by your portfolio is relatively proportional to your total files online, you can see that adding 10% a month to your portfolio will add about 10% to your sales, too, over time.
So far, I've tried to practice what I preach. The exception is that I couldn't sit tight and do 10, 11, 12 in the beginning and quickly ramped up to 50 a month until I hit 500, then have been trying to add 10% a month. It is getting harder and harder, but by increasing uploads each month I've gotten to almost 1000 uploads (and 1000 sales) in just under a year. It gets harder and harder, of course - I 'have to' add 100 photos a month now to add 10%. I'm OK scaling back to 5% a month, but the point is to keep maintaining growth over time.
My advice is to keep increasing each month while you try to hit targets of 500, 1000. Trying to reach a target is good, but doing it while increasing your uploads each month will serve you even better over time.