What percentage of phds in math actually get a tenure track academic job?
No. No one single factor guarantees you an academic job in a top university.  Whether or not you land such a job is a combination of many things.  These include, 


talent 
hard work
motivation 
quality of research 
quality of teaching
ability to network and get along with people
ability to communicate (both orally and in writing)
success in securing external funding
luck.


If you want such a job, here's what I recommend.  Choose an area that you're passionate about, go to the best school (most challenging and "highest rated") that you can get into, and work with an adviser with a strong publication record.  At each step along the way, surround yourself with (and learn as much as you can from) the most successful people possible.   

You can find a partial answer to your question about percentage by reading the annual Survey of the Mathematical Sciences (by the American Math Society):

http://www.ams.org/profession/data/annual-survey/annual-survey