The business and the rentals are definitely two QB company files.
Since you already own the rentals, the "multiple LLC" question is moot unless you plan to transfer title with the county clerk (or whatever it is in your jurisdiction). If you are carrying mortgages, the bank won't let you do this.
In my experience, having multiple LLC's is usually more work than property owner/managers want to do for the marginal bit of asset protection it provides. If you are the property manager, then YOU will be sued personally for YOUR personal actions, and multiple LLC's might not afford your other properties the protection you think. Easier and cheaper to just buy more insurance. I'm not an attorney, and I can't practice law, so you should definitely get a consultation before you proceed.
Anyway, if you aren't going to do all the work necessary to make the multiple LLC's work, thereby negating the benefit, then you should just keep things simple and have all the rentals in the same QB company, then set up the properties as "class" so you can have the profit/loss report by class (property).
If you have multiple entities, then you should have a QB company for each entity. The problem isn't the profit/loss, it's having the balance sheet comingled. It's a "Which LLC owns the cash?" problem. It's almost impossible to keep multiple entity balance sheets in a single QB file.
Mark Wagner, CPA
