It happened to me only once, in June 2016 on a A330 (one of the last ones). All 12 seats for myself ...
But, this is not as pleasant as an empty Y cabin, where you can lie down across 4 seats while you were expecting to be blocked by two bodybuilders during 10+ hours.
The most you can do is to flatten another seat to have an independent bed, as we used to have on LH B747.

The biggest advantage of an empty cabin is that you can have as many dishes / drinks as you want :-) .