To burn a selected group of photos in an event, create an album of those photos then burn the album. Remember that the combined size of the photos your are burning should not exceed the capacity of the disc you are burning.
iPhoto '11: Alert message appears when burning a disc with selected photos from an event
In some instances when burning a disc in iPhoto ’11, the disc is ejected and an alert message appears before the burn cycle is completed. iPhoto ’11 attempts to burn the entire event instead of just the photos selected in the event. The alert occurs when the size of all the photos in the event exceeds the capacity of the disc being burned.