The number of postcards you can send (ie., have simultaneously traveling) to other users depends on the number of postcards you have already sent. This encourages new users to test the system, while allowing more experienced users to send more postcards.
The limits for the number postcards you can send are:
This means that, for instance, if you have sent 21 postcards, you can have at most 6 postcards traveling simultaneously to other users.
To be able to request more, some of the postcards you've sent should have either been received or timed out in transit (expired). Postcards expire after travelling 60 days, although they can still be registered after that.
Note: the alteration in the number of postcards that a user can send is a recent feature that is currently being tested. Adjustments to these limits may occur in the future.