Do I need an LLC per store?

Yes, you can have more than one business under one LLC if all your businesses are in the same niche. Rather than creating an entirely new LLC, you can set up a DBA (Doing business as). For example, you have a fashion designing brand "Distinctive Fashion designer", but you want to set up an online clothing store " Distinctive clothing store." You can just file a DBA for the online store instead of creating another LLC.

The downside to this is that if a lawsuit is filed against one of these stores, the assets of both stores will be at stake. This is why many business persons will prefer to create a different LLC for each business, to protect the other in case of any lawsuit.