This type of tournament starts as soon as a fixed number of people have signed up for it. The number of required players range from two to hundreds.
Sit & Go tournaments of all variations are available 24/7 and have a fixed prize pool (since the number of participants is known in advance). As soon as one specific Sit & Go tournament starts, another one of the same format immediately becomes available. You can play multiple Sit & Go tournaments of the same format simultaneously. You can always unregister from a Sit & Go as long as it hasn't completely filled up.