Question for QA- What you are doing if your current sprint is 2 week and you have 20 stories, suddenly PO added more 20 stories how you handle this situation..?
If you are following Scrum as defined in the Scrum Guide, the Product Owner cannot simply add stories to the Sprint Backlog. The Sprint Backlog, which is created as Sprint Planning as a negotiation between the Product Owner and Development Team while considering past performance and forecast capacity, is owned exclusively by the Development Team.
If work comes in and it is a higher priority, there needs to be a discussion between the Product Owner and the Development Team on how to adjust the Sprint to balance the higher priority work with the remaining time and capacity of the Development Team. If the Sprint Goal is obsolete and the work that the team has been working on is no longer relevant or valuable, cancelling the Sprint is an option. If there are multiple Scrum Teams involved, dependencies must also be considered. The Scrum Master can facilitate any discussions or decisions made, including escalation beyond the Scrum Team if necessary.
Going back to the Scrum Values, the Development Team needs to have the courage and openness to push back against the Product Owner and protect their time. Going back all the way to the principles behind the Manifesto for Agile Software Development, development needs to be sustainable and done with a pace that can be held indefinitely, with continuous attention to technical excellence. Overloading Sprints or forcing the team is not collaborative and introduces a lot of risk and pain to an effort.
is the 20 old stories represent a release? if yes there is a real problem at the business vision, where that the 20+ stories will completely lead to another product and how these extra stories were added it should be a collaborative process (PO with team), you have to re-estimate the stories, re-planning the release and the iteration, some people make the iteration time dynamic so you can extend it from 2 up to 4 weeks according to the PO business values
in Scrum it is so important that all team members sit with PO in "Stories workshop" with "brainstorm" term to gathering all stories as possible this lead to good estimation for release, in your situation you must collaboratively re-planning the release(story workshop then estimate then prioritize and re-set the sprint time, finally assign the user stories in iteration according to the team velocity)