to reproduce (exact numbers are not important):

create DigitalOcean cloud in Jenkins with 10 droplets limit.
create 10 different images under that cloud
set limit for every droplet == total droplets limit
start a lot of build jobs to utilize all the images in the same time (say 10 jobs per image)

result:
jenkins perform queries to DigitalOcean API all the time to list droplets, create new droplet and so on. it leads to "API rate limit exceeded" error and inability to delete idle slaves automatically.

capacity per image is set == total cloud capacity because we want to be able to use the whole cloud capacity with every image created.

possible reason: digitalocean-plugin wrongly checks the # of running slaves and perhaps has a bug in logic.