My baby is only taking half an ounce during his feedings... idk if hes getting enough or not

I was told that they should eat 10-15 ml 20ml max on the first 2 days. Thats how much my baby was eating, by like day 4 he was eating 1oz then by the 1st week he was eating 2oz and now he is 3 wks+3 and eats 3oz.