I know how to switch from one virtual desktop to another, but it happen to me often that I open a 3rd virtual desktop not on purpose and even by doing a Super+Space I cannot remove it.
I can see 3 virtual desktop at the right side of my screen even if I use and want only 2 of them.
Do you know how can I delete the (n+1)-th virtual desktop ?
This is how dynamic workspaces work by default in Gnome Shell and Ubuntu with Gnome Shell. There is always one extra workspace over these that you are currently using, ready to switch to and use if needed.
You can change to a more traditional approach where you have a fixed number of workspaces. You can configure this in gsettings, but easiest will be to install Gnome Tweaks. On the workspaces tab, you can configure how you want workspaces to be handled.