You're right when you say that the quantity is important.
The UK talks about "units" of alcohol. One unit is 10ml of pure alcohol. Drinks in the UK are labled with their Alcohol by Volume (ABV) percentage. To work out how many units are in a drink you multiply the serving size in litres by the ABV number. Current UK guidelines are no alcohol at all if you're pregnant, and less than 14 units a week with some days alcohol free and don't save up the units for a binge.
In the past wine used to be weaker and serving sizes were smaller. One unit of wine is one glass at 125ml at 8%. In the UK by law everywhere selling glass by the wine has to offer a 125ml serving. That doesn't have to be a prominent offer though, it could be a footnote to the menu. UK serving sizes tend to be 175ml (for a small) or 250ml (for a large). This means there's huge variation in the quantity that people drink when they describe "5 glasses a week".
Five glasses * 125ml * 8%ABV is 5 units.
Five glasses * 175ml * 12.5%ABV is 11 units.
Five glasses * 250ml *12.5%ABV is 15.5 units.
15 units a week isn't hugely worrying, but it's a big difference. I guess we'd want people drinking 15 units a week to unwind after work to take up some exercise or mindfulness instead.
There's some cultural variation too. When a middle class couple in the UK talk about "a glass of wine" that could mean "half a bottle", and "a few times a week" could mean "most days". I used to work in a factory and people spoke about "going for a beer" after work, and that never meant one beer, it meant a drinking session where two or three beers are drunk.