Hello Nannab,
It's fine to have alcohol with Metformin. As a diabetic it's not a good idea to exceed the advised normal guidelines - 21 units for men or 14 units for women and I believe it's advised to keep below the guidelines. Commonly, I think we often under-estimate what we drink - say a "glass or two" would often really be 3 or more, or a pint would be two. A glass of red wine is about 1.5 units, so three glasses would be over 4 - you see how it builds up.
It's not the same for everyone, but 1 or 2 glasses of red wine can lower your BS. White wine or beer can increase it but always ensure you have some carbs at the same time - crisps, bread or something.
Good luck,
Russ