I can't comment about the minimum number of stores, but I do think its the correct idea. I live in a town of ~12000. We have 6.5 bookstores (including the good local thrift store as .5). Stores/restaurants do turn over fairly regularly, so its still tough, but it certainly seems viable. We get a good number of tourists, which helps but I do think you need a mix of restaurants, and a mix of different types of stores. Even as you go to nearby towns with big box stores, they all have downtowns that are doing ok with locally run businesses. Notably the downtowns all are small business focused with few if any box stores