Number of items per Sprint
Some teams count the number of items they can deliver to Done in a single Sprint.
Story Points per Sprint
Many teams use the Planning Poker technique (an adaption of Wideband Delphi) with a modified Fibonacci scale or “t-shirt sizes” to assess the size of work.
Add up all of the story points for items that have achieved the Definition of Done by the end of the Sprint. This is their velocity for that Sprint.