Some think that this is kind of a gray area at present. Optimally, and the current obstetrical recommendation, is to abstain from all alcoholic beverages. That being said, there is the frequently quoted (and misquoted) as 2 drinks a day being "good for you", or the anecdotal story of some Frenchmen who drink red wine and have virtually no coronary artery disease. The people who "need" an excuse to drink latch upon these stories like a drowning man clutching at straws. Any kind of regular use of alcohol is discouraged. (Like, "I just have a couple of drinks every Saturday to wind up the week", or "I need a drink after a hard day at work".) Is anyone saying you absolutely must not drink anything during pregnancy? Yes, some authorities say so. In a matter of practicality, a couple of drinks during the whole course of the pregnancy is not going to hurt you or your baby.
Turning to the baby, remember that anything that you ingest goes straight into the baby's bloodstream. A baby has a very low weight, so a drink you don't even feel has a great effect on the baby. Most especially, his (her) brain. Alcoholic mothers frequently or nearly always give birth to infants with some degree of fetal alcohol syndrome. This is thought to be due to the more or less constant exposure or "bathing" of the brain by alcohol while growing. It is obvious that this cannot be good for the developing brain. (When the brain is removed from the alcohol, as at birth, it goes into withdrawal. This sudden lack of the constant sedative effects of alcohol leads to the convulsions seen in FAS. And the baby will have some degree of birth defect, seen or unseen, and the degree is unpredictable.)
All this diatribe leads to a simple answer to your question: The best amount of alcohol to consume during pregnancy (or, for that matter, any time), is none. But, if you do happen to have a drink or two very infrequently (say monthly) during pregnancy, you are not hurting yourself or your baby.
If you have any problem with not drinking for a period of 9 months, then you may have other problems unrelated to pregnancy that need to be evaluated.