The maximum number of entries is stated on each competition so your chances of winning will vary from competition to competition. As an example, if entries are capped at a maximum of 3000, this means that if you purchase 1 entry and get the answer correct, your chances of winning will be no worse than 1 in 3,000. 

You can increase your chances of winning by purchasing more entries. For example, if you purchase 10 entries in the example above, your chances of winning will be 1 in 300.