Either option is an acceptable solution starting out.

Without knowing more about your project or the services you’ll be running or the traffic or load you’ll expect it is difficult to give you a good recommendation. If your primary concern is cost then you could either use a single droplet to start out or a few small droplets, I would not recommend running your db servers on anything smaller than a 1GB droplet however. Your microservices could likely be run on 512MB droplets. This is of course assuming a relatively small amount of traffic.

Using multiple droplets would make scaling a bit easier as you could then resize individual droplets as needed to increase capacity.