The fact that you are asking this question probably means for you, the answer is zero, but I’ll go ahead and answer anyway. You should always drink responsibly, whether at a business meeting or when out with friends. I realize “Mad Men” helped resurface the idea of the three-Martini lunch, but it’s worth noting that Martini glasses were much smaller in the ’50s and ’60s than they are today, and these lunches are also well known for having resulted in a lot of bad behavior.
If you’re out with colleagues or clients, drink in moderation in order to put yourself in a position to be successful. The business world is small, and people talk. You never want to gain a reputation as the person at lunch or dinner meetings who’s a good time but also a bit of a mess. Use your best judgement in every situation and consume a number of drinks that is right for you. No one can give a blanket number that works for everyone.