You will (should - as long as everyone completes the entire swap) always receive your "own" postcard (the one you started) back. For this particular swap, we had a rotation set up between the participants.
Our postcard round robin worked like this:
1. Get four or five people who want to participate, then set the rotation, which remains the same for each found. (Assume you're Person A. You will be sending to Person B, who will send to Person C, who sends to Person D, who sends to Person E, who sends to Person A (you).)
2. You take a blank postcard and divide it into roughly-equal sections, one for each participant. (I used a piece of paperboard from a carton of file folders, but some people buy blanks or use watercolor paper.) Then you create a bit of art in the first section – painting, drawing, collage (this is a popular one).
3. Address your postcard to yourself, place it in an envelope, and mail to "your" person in the rotation. For purposes of this example, you would mail your card to Person B.
4. At the same time, Person E has started his or her postcard, and will be mailing it in an envelope to you.
5. When you receive Person E's card, you'll decorate the next section of that postcard, write a little note on the message/address side, place it in an envelope (many of us continue to use the same envelope), and mail it on to Person B.
6. Again, at the same time, Person E will receive, decorate, and forward Person D's card to you.
7. Repeat steps 5 and 6 until you have received and decorated Person B's postcard, which will then be complete. Mail the postcard naked (no envelope) home to Person B.
8. You'll receive your naked completed postcard from Person E.

I'm also currently in a swap with an open-ended number of rounds and the participants may change each time. In that one, I'll receive my card back when it's full, which is basically up to the discretion of whichever artist receives it and decides that they've added as much as can be added. I've never been in one like this before, so we'll see...