[nimish]

It's the german tank problem.

Serial IDs, with some light assumptions, leak information about the total count of items.

[BenjiWiebe]

Just pick a random number at the beginning, and start incrementing IDs from there. Like personal checks starting at 1000 so they're always(ish) 4 digit. Of course, maybe pick another starting number that's less obvious.

[sk5t]

That's not effective at all.

[nimish]

Still leaks count -- you do similar stuff to estimate the minimum and the maximum.