When you sign up for the first time you have Basic account.
Basic account is FREE but it allows only limited participant numbers as shown by the chart below.

Basic - up to 4 participants(or teams) per Single or Double Elimination Tournament.
Advanced - up to 16 participants(or teams) per Tournament.
Professional - up to 32 participants(or teams) per Tournament
Custom - up to 128 participants(or teams) per Tournament