There are 12 words that start with 'backp'.
| backpack | backpacks | 
| backpedal | backpacked | 
| backpacker | backpedals | 
| backpackers | backpacking | 
| backpedaled | backpedaling | 
| backpedalled | backpedalling | 
Note: These 12 words that start with 'backp' are found from the USA & Canada version of the tournament Scrabble word list.