Tablets are very small and they cannot contan more grams of glucose than their actual weight.
So if whatever type of tablet weighs 1 gram, it can't be more than 1 gram of carbs even if most of the tablet is glucose.
A negligable amount, unless you are on a very strict carnivore diet or such.
I take vitB12 daily and iron intermittently. Obviously I can't mention brands here, but I strongly recommend you source those without additives, bulking agents or preservatives. I have to keep my vitamin supplements refrigerated because they are pure, but iron is less sensitive. You can't overdose B12 as you just excrete the extra, but it's easy to get iron wrong, so go carefully.