I found some more details about last year's competition:
"According to the foundation, an all-time record of 2,436 students entered the 2011-2012 competition and 1,541 projects were submitted."
And "Across the nation, 317 students were named semifinalists. This number was then narrowed down to 96 regional finalists comprised of students from 21 states."
Source: Silver Chips Online : Blazers recognized in Siemens Competition