Anomalies start respawning with late DA (1.7+, don't remember exact version number).
It's pseudo-random, and you probably won't notice it in the campaign. Most things in the campaign are fixed-it wouldn't surprise me in the slightest if anomalies were as well (within the campaign).
If you have enough survey ships out and running about, you can still run out of them (temporarily), at which point you'll notice the slow spawn rate, but it's not terribly slow, either.
They're not one at a time, IME, but they don't tend to spawn in large groups, either.
