Let's starts with some facts and numbers. There is five of us in the team. In the last three iterations, we completed the scope worth of 9, 13 and 12 story points. We feel comfortable commiting 10-11 story points in an iteration and we never commit more than 12.
If Harry joins our team next iteration, will you comfortably commit 12, on the average?
No. We may be delivering 12 from then on, but the reality is we don't know. It may be 12, but odds are it will be 14 or 8.
If Jack leaves your team next iteration, will we comfortably commit about 8?
Not necessarily. We may still be able to deliver functionality worth of 10 story points. But that's not all. We are not sure how you will react to that, but without Jack we may be able to deliver 13 or 14 pretty easily.
So you are telling me that arithmetic does not work in Scrum?
No, it's better to put it this way: arithmetic does work in general, but it does not properly describe a complex system, such as a Scrum team working on a project.
So what is the real message behind all of this?
The fact is that our team of five great engineers delivers about 11 story points each iteration. You cannot scale it, you should not theoretize about what the velocity would be, if we switched from 3 weeks to 4 weeks' iterations. You can't know what this number would be, if two of us left our side activities and focused only on the project. You can't know what this number would be, if all of us worked overtime We can commit to delivering 10-11 points each iteration and that's the only fact.