I know for certain it's whatever you care to contract for, but rotation between tasks is common.

A lot of these suppliers provide on-demand workers - if you need 40 man-hours of work on a one-off task, they can put 8 people on it and get you results within 5 hours.

On the other hand, if you want the same workers every time, it can be arranged. If you want a fixed number of workers on an agreed-upon shift pattern, they can do that too.

Even when there is a rotation, the most undesirable tasks often pay a few bucks extra per hour, so I wouldn't be surprised if there were some people who opted to stay on the worst jobs for a full shift.

Having tried both strategies, unless your task is brain-dead simple and/or you have a way to cheaply and deterministically validate the labels, always pay to retain the team.

Even if you can afford only a couple of people a month and it takes 5x as long, do it. It's much eaiser to deal with high quality data than to firefight large quantities of slop. Your annotators will get faster and more accurate over time. And don't underestimate the time it takes to review thousands of labels. Even if you get results l in 5 hours, someone has to check if it's any good. You might find that your bottleneck is the review process. Most shops can implement a QA layer for you, but not requesting it upfront is a trap for young players.