Here are some thoughts from Ralph Taylor on 12-marker Matches:
About 12-marker matches, there isn’t a lot to say. There are only three possibilities:
A perfect 12/12 means you share a common ancestor. But that ancestor may be many centuries in the past. The 90% probability level includes the past 60 generations (dating to as early as 510 AD); the 70% level includes the past 31 generations (as early as 1235 AD; and the 50% level (as likely to be low as high) includes the past 18 generations (as early as 1560 AD).
11/12 is inconclusive. Comparison of more markers is needed to draw any conclusions. If you are the one with only 12 markers tested, you should upgrade to more markers.
10/12 or worse excludes sharing a common ancestor within several thousands of years.
That’s cold, but true. 12 markers are considered sufficient to exclude sharing of a common ancestor, but not to include it.
Now, Lalia's interpretation... This means quite simply that if you match someone with the same surname 12 for 12, it suggests that you share that surname. However to trace your ancestry, you are going to need to test more markers and match more markers! (About which we will have more information in a future blog.)
If you do not share the same surname, or a variant of the surname, then a 12/12 match means that you likely come from the same general gene pool, such as the British Isles, and means nothing more.
We will have some forthcoming blog posts about higher levels of matches.
Good luck with your own genetic genealogical projects!