Why does a clock have 12 numbers?

because there are 2 halfs in a day and there are 24 hours in a day so half of 24 hours = 12 hours

People had clocks before they had zeros. The Babylonian number systen was based on 60 not 10 or 100. It was based on 60 because you can divide a lot of things by 60. You can divide more things by 60 than 100. So if you do not have a place value system with zero invented yet 60 is a convenient number. There are 5 twelves in 60. Also you have 10 fingers if you count your thumbs. You have three lines on the inside of your four fingers which adds up to 12.