PMP Practice: Continuously assess and refresh team skills to meet project needs

Kit Abernathy-Osei is leading a hybrid project at Magnificent Detour Studios to develop a mobile puzzle game. The predictive design and art phases are complete, and the team is now entering agile development sprints for coding and testing. Kit has a fixed budget and needs to determine staffing requirements for the next four months. The product owner has prioritized 120 user stories in the backlog. The development team has never worked together before, so their velocity is unknown. Kit needs to forecast how many developers and testers to bring onto the team. Which approach would be MOST effective for determining resource requirements at this stage?