For most people, one LLC is enough. But sometimes it makes sense to have more than one LLC.
If you own major business assets like equipment, warehouses, machinery, vehicles, etc., then you might want to set up another LLC and transfer your business assets to it. The LLC that owns these assets is called a holding company. Your other LLC that does business with the public is called an operating company. Your operating company can then lease the assets from your holding company for market rates.
Why would you want to do this? If someone sues your operating company, the assets in your holding company are protected from that lawsuit.
There’s another scenario where you might want to have more than one LLC. That’s when you have two different lines of business. If you have separate divisions, do business in separate locations, run businesses in separate industries, then you might want to set up separate LLCs for each line of business. Then the liabilities in one operating company are protected from the other operating company.
There are some drawbacks to having LLC holding companies and multiple LLCs. You must treat each LLC as strangers. They must each keep separate books, separate bank accounts, separate employment agreements, etc. If they share overhead, then they need to have a shared services agreement or an inter-company agreement between the companies. You need to have documents transferring asset ownership to your other LLCs such as a bill of sale, warranty deed, assignment, etc. Then you need to have some sort of rental or lease agreement between your LLCs.