Each division of your competition has a Max Registration cap associated with it. This value determines the maximum number of registrations you want to allow for the division. Once this cap is reached, the division will appear as being "SOLD OUT" to users at registration, and they will be unable to register for it.
You can set the number of registrations allowed in one or more of your competition's divisions in the "Programming" section of your competition dashboard.
Please refer to the related article "Modify existing divisions" to learn how to do this, being sure that you specify the appropriate Max Registrations value when editing the division you'd like to adjust.