Step 1: Identify Total Story Points in the Backlog
From the backlog table:

Total story points = 6 + 4 + 8 + 4 + 21 + 7 + 5 + 9 + 3 + 5 = 72
Step 2: Determine the Team's Velocity
The risk register mentions a burn reduction of 1 story point per sprint from sprint 4 to sprint 6 due to a holiday schedule. However, assuming a normal velocity of 5 story points per sprint, we can estimate sprint completion.
Normal sprint velocity: 5 story points per sprint
Reduced velocity for sprints 4-6: 4 story points per sprint
Step 3: Calculate Total Sprints Needed
First 3 sprints: 5 points per sprint → 5 × 3 = 15 story points completed
Sprints 4-6: 4 points per sprint → 4 × 3 = 12 story points completed
Remaining story points after sprint 6: 72 - (15 + 12) = 45 story points
Remaining sprints needed at normal velocity (5 story points per sprint): 45 ÷ 5 = 9 sprints
Total Sprints Required
Sprints 1-6: Already planned.
Additional 9 sprints needed.
Total sprints to complete the backlog: 6 + 9 = 15 sprints.