How many villagers do you need in Age of Empires?
You should have at least 20-25 villagers before clicking the Second Age. You can also have 25-30 if you want to advance faster from the Second to the Third Age later.
To reach the Third Age, generally 24 to 30 villagers are suggested.
For the 4th Age, it depends from your strategy. 30-35 villagers might be enough if you want to click quickly the fourth age.
If you are planning on booming (improving your economy strongly) you can have a range of 50 to 100 villagers, based on your needs.
In particular, the number of villagers depends a lot on your build order, which also vary between Age of Empires 1 and 2, or other chapters of the saga. You can see in this website an example of a Build Order for Age of Empires 1: How to reach the Bronze Age quickly.