At the Medical Writing, Editing & Grantsmanship blog, this post notes the rising estimates of Challenge Grant applications. "Although the dust has not settled as yet, the NIH has currently accepted more than 18K RC1 applications, with another 11K in the error correction queue," according to the blogger. "CSR expects to engage 10K - 15K reviewers" -- which explains why you've probably gotten more than a few invitations already.