TimeToSignUp has an option for participants to select a number when signing up:

To enable the number option for sign ups:

  1. Log in as the administrator
  2. Go to the sign up sheet
  3. Either A) add a new column or B) edit an existing column
  4. On the column setup/edit screen, if necessary, click on "Show Advanced Options"
  5. Choose "Number" and if desired, select a maximum number permitted for the column. The maximum is for the entire column, so if you are permitting a total of 80 participants in the column, select 80. Be sure to save your changes!