There is a great Nomination FAQ sticky under the nomination tab.
As FMB indicates, it is certainly possible to achieve noms to different academies from the same source. But, often, in very competitive districts, the member makes you rank your choices, and then gives the USNA noms to the kids that want Navy #1, and so forth. That way, the member can give 30 nominations to 30 different kids assuming the member had 3 slots that year, one at each academy.