Drone parts are required at major upgrade intervals. Here are the counts for upgrade:
Unsure at the lower levels...
Level 80 - 300 parts
Level 85 - 350 parts
Level 90 - 400 parts
450 parts
500 parts
600 parts
Level 110 - 700 parts
800 parts
1000 parts
Level 125 - 1500 parts
Level 130 - 2500 parts
Level 135 - 3000 parts
Level 140 - 4000 parts
Level 145 - 5000 parts
Unsure beyond....