Since you have Session-Based Test Management, you can use your test sessions data to show your team's testing capacity.
For example:
You can plot the number of sessions your team did throughout time. Downtrends may intigate your team is having problems to keep a steady pace or that your testability has improved, demanding less test sessions for the same results.
Normalizing these data with other team capacity metrics, such as number of people involved or story points (if you can very consistent and mature with them) may make this chart more precise.
- You can also breakdown the type of work done in each session, to tell a story about issues your team may face with testability and reportability.
You can find more details on James Bach's article on Session-Based Test Management.
On a final note: Don't count lines of code or number of bugs. Use any data or chart you have to tell a story about testing. Focus on creating adaptability, optionallity, and experimentation. Identify what issues you have and see coming, and prepare yourself to remove those impediments to good testing. You may identity that you gonna need to put more effort in setting up better code quality analysis, or solve communication issues, or test labs issues. These problems may be solved be hiring people with new skills, or changing focus, or changing roles inside the team. You will know what works best to your particular context by experimenting, throughout time.
