All sites run qualifying tournaments which guarantee a certain number of qualifiers. Depending on the number of entries, the number of qualifiers can increase. If for example there are 15 qualifying spots available, as soon as the player on the bubble standing in 16th place is eliminated. all 15 players remaining will get a message saying they have won and qualified.
It is important to keep track of your position when the tournament is getting close to the bubble position and monitor your decisions closely.