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So if I am still waiting for one more LOR to be checked in, but have the required number for the school met, can I assign all the LOR to the schools, and have the school start reviewing the completed app with the letters that are there or willthey not start reviewing until everything is there

The school will consider your app "complete" and begin reviewing it as soon as you send the amount they require. If you want them to wait until you have all your letters in, wait until that time to submit your app.