What amount of alcohol consumption is beneficial? Doesn't any amount of alcohol consumption kill brain cells?

Depends. There is nothing called safe drinking. For male, more than 3 drinks per day and for female more than 2 drinks per day can cause liver damage in the long run. Certainly, other pattern of drinking, such as bing drinking can cause acute liver problem.
Very little. It is generally agreed that 1-2 alcoholic beverages per day is safe. The benefit is still debated, and is likely limited to only red wines which contain tannins. I would not recommend drinking any alcohol to achieve health benefits, but rather avoid excess to prevent adverse health outcomes.
One glass of wine . Usually a glass of red wine daily is recommended for heart health. But everyone has different tolerance to alcohol. Alcohol is a direct toxin to the liver, thus one glass of wine, a shot of hard liquor can cause a mild inflammatory effect. Moderation is usually the best recommended amount.