Before we talk about the number of words, let's talk about the different between a milestone and an average. The average is well the average. You take all the words the children have and then you divide it by how many children there are. Milestones are a little different. A milestone is when 90% of children at that age are doing this skill/task. So for vocabulary, it means that 90% of children already use that many words and more. So when you see a "milestone" for vocabulary, that's not the average, that's what the majority of children are already doing at this age.
Another way of thinking about it is percentiles. If you children is not meeting a milestone, it means that they are in the 10th percentile (or bottom 10%) for that specific skill.
Why am I talking about milestones, percentile, and numbers? Well, it's important to know the significance of milestones. Although milestones are not the only consideration when completing a speech and language assessment, but it does signal when one should seek a consultation to see a speech-language pathologist.
Now I'll get to the vocabulary milestones. Wordbank is a great resource that documents vocabulary develop in children who are speaking a variety of language. The following numbers are pulled from a sample of children who speak English in America.
These numbers change depending on what language your child is exposed to. Is your child learning a different language? Let me know! Let's see if we can find the numbers that are relevant for your child.
Source: http://wordbank.stanford.edu/