Learn English – Why do we brew beer but make wine

I believe that they are collectively called "promotional models" (or simply "promo models"). In the case of beer, wine, energy drinks, etc., they appear to be referred to as "Beverage Promotional Models". Depending on the brand that they are promoting, they might also be called "Red Bull Girls", "Miller Lite Girls", "Bacardi girls", and so on.

Promo girls are not necessarily restricted to bars and are usually seen at launch parties, expos/trade shows/conventions and the like. Their customary presence in conventions has led to them also being referred to as "convention models".

'Beer' can mean both the substance "beer" and the (standard or more arbitrary) units by which it is measured ("a beer"; "two beers").

'Beer' referring to the substance cannot be plural (or singular) (ie. it is non-countable, see comment).

But when used to mean the units by which you buy / consume the substance (eg. "we each had 3 beers last night"), 'beer' is countable. See this (Canadian) article at Ask the English Teacher and this comment in particular, for more clarity.