Unless it varies by county or something like that, it is 10% of your CRP acres that you can plant to food plots. Dbltree already mentioned the NRCS office, I second that. You can clarify this question but you will also likely have to revise your conservation plan and specify where your food plots will be on an aerial map, etc.
Also, a key thing I learned in the past 6 months... you can plant clover and alfalfa, provided you do not harvest/bale it of course, on all of your CRP acres. So that can really expand your options. Again, check this with your NRCS office.