回复：What you mean USJMO is selected by top certain No. of people

Here it is: 

Approximately 270 of the top scoring AMC 12 participants (based on a weighted average of AMC 12 and AIME score) are invited to take the USAMO. Approximately 230 of the top scoring AMC 10 participants (based on a weighted average of AMC 10 and AIME score) are invited to take the USAJMO - See more at: http://www.maa.org/node/79/#USAMO