When someone posts a story with the message “Send Numbers” on Snapchat they are asking their followers to send them a random number so that they can rate you anonymously in their story.
For example if you reply to a “Send Numbers” post with “49”, then will then post a story with the number “49” on it that rates grades aspects of your appearance or personality, or answer questions about you.
In the image below you can see that the individual has used this method to answer questions by marking their answers with a red dot.
Whoever the number “432” is will know a bit more about this Snapchatter’s feelings towards them.
Replying to a send numbers snap will give you insight into their thoughts on you without anyone else who sees the story knowing that the story is about you, as you will just be an anonymous number.