If a task pool consists of several hundred tasks, then 10% of the pool should be control tasks. If the pool contains thousands of tasks, just 1% is enough.
If possible, classes should be represented in the set of control tasks in proportions similar to the classes in the general pool of tasks. These proportions are set by the requester. Suppose you need to determine the type of accommodations on a hotel aggregator website: family, business, casual or luxury. Luxury accomodations make up just 10% of the main pool, but performers see them in every second control task. As a result, you will not be able to check whether the performers correctly mark the other types of accommodations, and you risk getting poor results with noisy data.
Tip: If the quality or labeling speed is low, check the control tasks. Perhaps they contain incorrect, outdated or unclear examples.