The letter outlining the charges to Lance basically says USADA has numerous teammates who will say he was dirty.
If you are too lazy to read it, let me quote the key parts when they list what they have on Lance
The document alleges that Lance was part of a huge conspiracy to commit doping led by Mr. Bruneel, Dr. Celaya, Dr. Del Moral, Dr. Ferrari, and Mr. Marti.
With respect to Lance Armstrong, numberous riders, team personnel and others will testify based on personal knowledge acquired either through observing Armstrong dope or throught Armstrong's admissions of doping to them that Lance Armstong used EPO, blood transfusions, testosterone and cortisone during the period from before 1998 through 2005, and that he had previously used EPO, testosterone and hGH through 1996.
Numerous riders will also testify that Lance Armstrong gave to them, encouraged them to use and/or assisted them in using doping products and/or methods, including EPO, blood transfusions, tesosterone and cortisone during the priod from 19999 through 2005.
Representatives of USADA have interviewed Dr. Martial Saugy, Director of the Lausanne Anti-Doping Laboratory which analyzed the urine samples from the 2001 Tour of Switzerland. Dr. Saugy stated that Lance Armstrong's urine sample results from the 2001 Tour of Switzerland were indicative of EPO use. Multiple witnesses have also told USADA that Lance Armstrong told them he had tested positive in 2001 and that the test result had been covered up.
Lance Armstrongs's doping is further evidenced by the data from blood collections obtained by the UCI from Lance Armstrong in 2009 and 2010. This data is fully consistent with blood manipulation including EPO use and/or blood transfusions.
Lance is the only rider who USADA is going after in the group. They went after him and not his teammates because "every other rider contacted by USADA regarding doping in cycling agreed to meet with USADA and to truthfully and fully describe thei involvement idoping andall doping by others of which they were aware" but Lance did not.
As for the # of witnesses, they say it is more than 10 cyclists. They also say only info was used that was based on first hand knowledge.