You can change the number of seats available in your car in 2 ways. You can either do so when creating a new ride or editing an existing ride.
While creating a new ride:
After you have entered your destination address and picked a time for your ride, you are taken to a ride overview screen in which you can modify the number of available seats you have (as shown in the 3rd image below).
Editing existing ride:
1. Click on the three-dot menu field on the ride you would like to edit (see the 1st image below).
2. Click "Edit"(see the 2nd image below).
3. In your ride details click on the pen-button next to "Available seats"(see the 3rd image below).
4. Change the number of seats you would like to have and click "Confirm"(see the 4th image below).
Once you reach the maximum number of confirmed passengers (your predetermined available spots), then you will no longer be shown as a prospective driver to other users (you will no longer receive passenger requests). If one of your existing confirmed passengers cancels the trip then you will again be visible to other potential passengers.