Just in case someone else ends up here from a google search, I can verify that this behavior of being limited to 5 workspaces is still a problem. However, @AntiqueAntelope 's fix of digging into the settings, finding the sidebar player toggle and flipping it will solve your problem.
Specifically, here are the steps:
Open your preferences
Search for "Enable Player"
This should pull up only the toggle you need.
Switch it "OFF"!
4.5 IF the toggle is already off, toggle it "ON" and then "OFF" again!
Add a new workspace and it should appear and stick like it should.
Open tab should be highlighted with much visible silver tone colour, keep closing wrong tab, very annoying.
Also "x" to close tab should have been on the right, why ? When the tabs open towards right, why on earth you'd put the close "x" on the left side ?
I love opera and the dark theme but these two things needs must be corrected.