As specified in the rfc 6585rfc 6585:
The 429 status code indicates that the user has sent too many requests in a given amount of time ("rate limiting").
The response representations SHOULD include details explaining the condition, and MAY include a Retry-After header indicating how long to wait before making a new request.
This specific header is only optional. So if you don't see it in the headers that's because it isn't there.
Including the condition in the response representation is only a recommandation (and so not mandatory)
There is no rules regarding this limit: it depends on the server configuration.
For exact meaning of verbs commonly used in RFCs : you can read thisthis