Regular Hellman's/Best Foods mayo has a few carbs. The FDA law allows manufacturers to print 0 on the label if the count is less than 0.5g. It's okay to count it as 0g carbs if you don't use more than the suggested serving listed on the label (1 Tablespoonful). If you use a cup of it in a recipe, though, then the carb count would be about 8g carbs.

You might choose "Salad dressing, mayonnaise, soybean oil, with salt" from the list. The Hellman's/Best Foods I use is made with soybean oil. Even for the kind that claims to be made with olive oil, the majority of the oil used to make the mayo is still soybean oil. Check the ingredients on the label of the type you use.