How can I limit the number of registration responses (spots) for something like volunteers or supplies needed?

Do this using the Registration Limit option for dropdown responses in registration attributes.

Note: If you have a single group of people that you are asking to volunteer for spots at an event, the volunteer feature is likely your best option because it allows people to choose a role and/or simply accept or decline. 

Using the Spots feature will help you recruit volunteers for events, gather needed supplies for mission projects, get the right balance of food categories for potlucks or other types of registrations that require limiting the number needed for each response.

Go to Groups & Events > Settings > Registration Attributes. 

1. Click the New Attribute button.
2. Select Dropdown for type of attribute, then give it a name and description.  It will default the sort order number to the bottom of the list of attributes.  Click save.
3. On the next page put in the Attribute responses (values) and find the Registration Limit column to set the limit of number needed.
4. When Saved, the attribute looks like this.  Note the Registration Limit column.
   
5. Add this registration attribute to the registration form.
6. When someone tries to register they will see the number still available from the question drop down.  Notices there is no longer a need for Ushers in this example.