I've been assuming the collection activities are based on overall population, with the target countries conducting collection sweeps as needed.  A hundred today might be fine, but a thousand more the following month might be required.  It's random...but iffy enough not to have a huge impact on public activities...mostly because if a large sweep is conducted, people will generally feel safer for some time immediately after, and the streets would be full of people out to enjoy the outdoors free from fear of being picked up.