Relay classes require the person registering the team to enter all team members' information at the time of registration. Because of the way relays are set up, you aren't able to edit the number of participants allowed in a relay once it has been created.
If you'd like to allow both the original relay team size and a second team size, you can can create a different class for each.
If you created this class by mistake and no participants have signed up for it yet, you can hide the incorrect class and create a new class with the correct team size.
If participants have already signed up, and you do not want to include relay teams of this size in your event, you can manually cancel each participant and have them sign up for a new relay. See How can members of a relay team transfer to another class?