You can contact their Support to get that increased. Just guessing at the reason, but if there was no limit, what happens if someone hacks your account and spins up a 100,000 node cryptocurrency mining farm?
Beyond anything, it tells people about their target audience, which is indie development. That's fine, and it's a great market to be in. But in the case I have to spin up 17 servers in 24 hours in three continents, I can't really afford to deal with DigitalOcean's support under that kind of stress. This doesn't happen often, but when it happens, it absolutely breaks you.
Thanks! Zach Director of Support, DigitalOcean
( I never knew there was such a low limit of 10 instances. )
I've been a DO customer for 5 years but I'm not sure when my droplet limit was increased.