How can I know how many questions/answers I have edited so far for a specific Stackexchange site?
With the new redesign of the Users Page, you can now see the number of edits users have made (I believe this includes both edits and retags).
If you need more detailed information, you can write a SEDE query to find out -- now that the PostHistory table is included in the data, what you want can be found out exactly. (At this time, however, I don't know of a query which correctly does the calculation.)
EDIT: Turns out I got impatient and wrote a SEDE query myself! http://data.stackexchange.com/stackoverflow/s/1397
There are SEDE queries that almost* do what you want. I put one together — as a learning exercise; it's probably not very efficient — based on some earlier queries, it's available at
* SEDE queries are imperfect because they only make the last editor's information available. So, if you edit a question and then someone else edits it some more, you won't get credit for that edit. They also assume that you get credit for both question edits and answer edits, which I'm not sure is true.