With the new information, I think I would change your form a bit in order to make the process more flexible in the future in case you add a bus to the fleet. I don't think you need to count the driver in the passenger total, since the question on your form shows that the passenger is not counted in the number of seats. At least that is how I interpret it with "+ Driver". If the driver is included in the number, I would change it to 15, 13 and 29 seats respectively and adjust the logic below accordingly.
First, I would change your first question to:
This way, it puts the work on the person submitting the form to choose the option that will meet their needs.
Then in your workflow, you can use a switch based on which option is selected. If you add a bus, you will just need to add a new case to the switch. Since I don't think you can handle the passenger limit check in MS Forms, we'll still do that in the workflow.
I would create a variable called passengers and make the default value the total of the students and staff. For what I tested with, this expression should work as long as you update it to your question values: add(int(body('Get_response_details')?['r9251ceba05194c7f88522399a853c494']), int(body('Get_response_details')?['r97434f401f49487caa16e6693c569d18']))
Then your workflow will look like this. The first condition will check to see if the total passengers is more than 30. If it is true, you can do what you want to notify the requester that the buses cannot hold more than 30.
Then the switch will go down the path based on the option selected Except for when they choose both buses, you will check option 1 and option 2 to see if the total passengers exceed 16 or 14 passengers. Again, you can determine what to do if it does. You do not need to check the number of passengers for option three when both buses are selected unless you want to check to see if the number of passengers is 16 or less, in which case you could do something to see why they need both buses when everyone will fit on one.
Granted, you'll need to do a check to see if the bus has already been selected for the date chosen, but this should get you started.
This is how the first condition is configured, and you would set up the other condition the same way:
