More seashells will spawn one after another over time on Sand blocks during your gameplay, and even faster whenever your player-character is in range of view.
This way you can also make seashells spawn on artificial biomes made of Sand, as long as Water is close enough. They occasionally also can spawn on Sand if no Water is near, but more sparely.
Apart from being fine decorations for your home (except if you also use blocks crafted from obsidian and blocks of sand, which might invite unexpected visitors, such as Rocksters, Rockzillas or Pebbles), you will need seashells to unlock the crafting recipe for Iron Mining Cells and to craft them.
Additionally to that, seashells are an ingredient of Opalescent Ceramic Wall blocks. You can find the crafting recipe for these small-tiled building-blocks in the Store as a part of the "Ceramic Pack" that you can buy if you want to support the game's development.