Velocity is not the number of Product Backlog Items (PBIs) a team can do in one iteration (this would only make sense if all PBIs had the same size which is very unlikely the case).

Velocity is the quantity of work a team can achieve in one iteration and is thus expressed in the same unit you use to estimate your PBIs, usually points.

Velocity is used as yesterday's weather method: "tomorrow will be like today". You measure the actual velocity (how much points did the team burn during the last iteration) and use this value to pick up and commit on as much PBIs (total size) for the next iteration.