velocity limited by one person

How the velocity of a team is established when it depends on the capacity of a person.  For example more developers than QA.  developers get free time while QA spends all their time on tasks?

@Byron España, Velocity belongs to the Development Team, not to individual members. Irrespective of the distribution of skills within the Development Team, together they are all responsible for producing a "Done" Increment.

Hope that helps.

If you are in fact bottlenecked by QA maybe your team should recognize that and find ways to eliminate that impediment.  As @Steve Matthew says, it is something that the entire team owns, is accountable and responsible for maximizing.  The team can find ways so that one person is not the gating element to delivery.  

Sometimes when one individual has a rare skill within a team, their role should shift (at least partially) to that of a servant-leader or coach; someone who empowers other members of the team to cross-skill, or ensures that they have the things they need to fill this much needed function.

For instance, I've seen QA engineers step back from writing all tests for their teams, and instead work outside of the teams, providing advice, feedback, and taking responsibility for the smooth running of the test automation framework. The other engineers took responsibility for writing automated tests, or testing manually when it was required. They knew they could ask the QA engineers for assistance when it was needed. Once the other engineers were less reliant on assistance with QA, the QA engineers took on other aspects of continuous improvement, such as helping to identify trends in reported bugs, providing greater transparency of agreements around testing, and helping raise the bar for the level of testing we were doing.

Testing is no longer a bottleneck for our teams.

How the velocity of a team is established when it depends on the capacity of a person.  For example more developers than QA.  developers get free time while QA spends all their time on tasks?

Velocity isn't established, it is whatever it is observed to be. Perhaps nothing is really being completed by the team at all.

When agile practices are attempted, such as measuring a team's capacity for completing "Done" work, it's transparency that starts to become established. In your case, transparency has been gained over the cost and consequences of skill silos which inhibit collaboration. That's the problem to be solved.