If you are in middle of sprint and stakeholder comes up with new set of requirements then product owner has to decide if those requirements need to add to product backlog based on their priority. If he feels that those requirements are too critical then he can cancel the current sprint and take up those items from the top of product backlog to the sprint backlog. Saving Changes...
Though your question is not very clear, I am providing some feedback.
The stakeholders can add n number of stories/features to product backlog and PO prioritizes for a sprint. This is an iterative process and can happen any time.
However, once a sprint is started it is not recommended to change stories accepted for a sprint. Saving Changes...
