Sure thing, Adrian. Just go to Admin > Field Management > New Custom Field. Make sure you select Score as the field type. Once that field is in place, you can use flow steps in Smart Campaigns to increment that field with each registration. Make sure you use the Change Score flow step and use +1 as the value. Then, you'll just need a separate Smart Campaign to look for when the data value in that scoring field changes to meet your registration threshold.
A couple screenshots below, hope this helps.