Opera is Chromium-based and on Windows at least, Chromium-based browsers use the system network stack. This implies that you have to edit the registry to change the max amount of connections. Of course, doing so will affect IE, Edge and all other browsers and apps that use the system network stack. (Firefox and Opera 12 use their own.)
So, something like https://www.mr2t.com/tweaks-ie-connections/. But, never tested to see if Chromium honors that. If not, things might just be hard-coded in Chromium.
Would be nice to have something in Opera's settings though if possible.
If you can find something that works for Chrome, it might work for Opera too.