If you place enough sand, does it start making desert mobs spawn in Terraria?
Yes, when you place enough sand in an area, then it turns into an artificial desert biome. But you need at least 1500(desktop, console, mobile) or 1000(other) blocks of sand. You can tell that you successfully created an artificial biome if you have a spot where the background and music change when you move your character there.
More information on this on the wiki.