If you have an LLC, you should know how an LLC protects assets. Accordingly, you should also know why having multiple LLCs is better when you own many properties. If a tenant, contractor, etc slips and falls and sues, they will have to sue the LLC that owns the property. If they win, assuming your LLC was properly formed and managed, the plaintiffs can't reach your personal assets (hooray for LLCs!) and their judgment can only reach what the LLC owns. Now, if your LLC owns 10 properties, they can reach those 10 properties to satisfy the judgment. What if the judgment is really large? If your LLC only owns one property to reach in the event of a judgment against the LLC.... I think you get the idea now.
Plenty of people own multiple properties under the same LLC, but many also choose to go with the added protection of owning only one or two properties per LLC to limit what can be reached. Does it cost more money to have more LLCs? Yeah! More asset protection = more money. Car insurance policies cost more money if you get better coverage with more protections, that's how things work. If you don't want this added protection, don't do it. Simple.