Race numbers are assigned separately per participation type, so you can set a specific participation type to assign numbers as people register. When you are editing the TeamRaiser:
  1. Go to step 6, "Manage Participation Types"
  2. Next to the participation type click "Edit"
  3. Go to step B, "Select Type Options"
  4. On step 4, "Race Number Generation", check the box next to "Generate Increasing Numbers" or "Generate Decreasing Number" and choose a number to start with. Click "Next" to save.
This will assign race numbers, using the scheme specified, to participants as they register for that participation type. Note that duplicate numbers can be assigned under particular circumstances, for more information look here. To assign a particular race number to a participant, please refer to this solution.