Space is big, take 2,000,000,000,000,000,000,000,000,000,000,000,000,000,000.
This may not be as effective as you think. There are also already 600,000 pieces of space junk of sizes 1-10cm. You've added few thousand <2mm pieces. And at that size, they won't need much abrading before they're harmless. (And the cloud will self abrade in the early phases.)
Or we can compare it with the hundreds of tonnes of rock hitting earth every day. We don't seem to have a good handle on this, but I just found one study suggesting a diurnal flux of 44 tonnes <100g particles. Much of that is on grazing, retrograde trajectories - as you can verify in any meteor shower.
There's also technical challenge of trying to disperse the cloud so it's not compact but doesn't end up too thin. And there's no chance to practice - because once it's done, everyone knows.
Biting the hand that feeds IT
