The essay is only 1/3 of your writing score; the multiple choice makes up the other 2/3 of the score.
I'm not really sure what your question is - I'm assuming you mean how many, and what percentage, of SAT test-takers get a 12 on the writing section essay. These tables reveal partial answers: http://www.collegeboard.com/prod_dow...ore-report.pdf
It appears that maybe ~12,500 students a year get a 12 on the writing essay from the graph (it looks like that bar is maybe a quarter of the way past 0 on the way to 50,000). Using the 1,376,000+ students who took the test in that cohort, it would appear that 12,500 is less than 1% of the students who took the test. Even if the number is 15,000 (a much more liberal and improbable estimate given the graph), that's still only 1.1%. So I think it's safe to say that ~1% of test takers got a 12 on the writing portion essay.
A 710 is in the 96th percentile, so I wouldn't say "only" a 710. That's a pretty good score.