just want to know is it possible to format used DV tapes? and how many times you can record over a single DV tapes before it degrades?

Tests were done recording over a short segment many times without dropouts here:

http://www.adamwilt.com

However, when I play back my DV footage, I notice dropouts after the 6th to 8th playback.

I don't think it's a good idea to re-use tape, unless it's footage you don't really care about. If you're shooting a wedding, on the other hand, I suggest using high quality, new tape.