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ISBN book numbers are sold in blocks of 10 numbers by RR Bowker. Just like the savvy bookseller can tell if your ISBN book number belongs to you or to a vanity press, they can tell whether you own 10 numbers or 100. Owning a larger number of ISBN's lets booksellers know you are serious about your self publishing books program.
It has been suggested by some that you actually start your first book on your 5th or 6th number and assign the first 5-6 to titles that you may or may not even be publishing to make it look like you have a "program," and are not just self publishing only one book or a single title.
*10 numbers is infinitely better that using a number supplied by one of the many vanity/subsidy presses.