Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
Number of interviews: 3
Skype interview: No
Length of interview: 15-25 minutes
There were three interviews with similar structure. In one there was a choice of topics and this led into a very specific discussion about the topic at hand.
The second interview was about something very abstract, testing the ability to think on your feet in a new situation.
The final interview was very mathematical and about sketching graphs and differentiating a simple equation.
I didn't prepare for the test.
Don't overly prepare. Just be passionate about your subject and enjoy the challenge that comes with the interview. I don't think there is truly a way to "practise" the interview