You could use a ‘count related’ lookup column to get close to what you need
to do this, you’ll need to split in to two lists: one called ‘devices’ and on called ‘reservations’
Now, create a lookup from reservations to device, so that when you make a reservation you can select a device.
Next, make a lookup from the device list to the reservations list. Make sure to select the ‘count related’ checkbox when creating the column.
This should give you the number of reservations for each device.