Hi Mario,
Great question!
Just as you can view your own Cards page at https://trello.com/my/cards, you can also view others Cards page by changing /my/ to their username. This will only show you cards that you're otherwise able to see, like cards on boards that you're also a member of.
Alternatively, if you upgrade to Trello Business Class, you do get access to a little more information to view each team members' cards across the entire team, and using that, you can see how many cards are assigned to each team member.
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