Linux tries to use as much RAM as possible, it doesn't matter if your WP sites get 20 or 20k visitor a day. In other words, this is not a metric to worry about.
You should, however, add swap space to your server, at least 2GB. Then keep an eye on it: if the swap usage gets continuously close to 100%, you need a RAM upgrade.
IMHO, DigitalOcean droplets are weak in terms of CPU power. Anyway, a magic number to have in mind is twice the number of CPUs as the top limit to server load (i.e.: 2 vCPUs = 4.0 of server load).
And the most important tip: optimize your WPs, avoid unneeded plugins, and make good use of caches (Cloudflare in front of your domains might be very useful too).