Marissa,
LLCs are great for holding long term properties. The LLC will allow you capture the tax benefits of your rentals based upon the flow through taxation of the LLC. Additionally, the LLC will help contain liability exposure that may arise from the property inside of the LLC and away from you on a personal level.
The key here is that LLC contains exposure, so any other holdings or assets within the LLC would be at risk of an injury arose on a property within the LLC. Typically, when you are holding residential properties it is often not necessary to have an LLC per property. While it is true that you would only have that one property at risk, you need to do a cost-benefit analysis in setting up one LLC per property. On the down side you have creation costs, initial filings fees with the Secretary of State and in most jurisdictions you have to pay the Secretary of State an annual fee to keep the business in good standing. If the LLC is not disregarded for tax purposes a tax return will have to be filed for each LLC. Each LLC will need to have its own bank accounts.
Every investor has his or her own level of risk tolerance. Even though I'm an attorney and can create LLCs with no effort I will typically hold 3-4 residential properties per LLC. I manage my risk exposure by keeping an eye on how much equity I have in the properties. If I have more than $250k in combined equity I will reposition one of the properties into another LLC. Many of my clients will not have more that $100k combined equity in their LLCs. Another factor to consider is to keep similar properties together by not mixing higher risk multi-units in the same LLC as a suburban residential holding.
A good strategy to help minimize risk exposure from the investments will be to have another entity (LLC or Corporation) to serve as the manager of the LLC. This management company would be the entity that interacts with the public, maintains the property, negotiates contracts, advertising, marketing, etc. Basically, the management company is where the business activity occurs.
In certain circumstances these different business activities could be separated into individual components within a Series LLC. However, before jumping into a Series LLC I would consider a couple of factors. Are series LLCs recognized in the state where the investment properties and business activity occurs? If not, then the Series LLC will need to be created in another state that recognizing the Series LLC. Since it is in another state you will have to hire a resident agent in that state and then you will have to file the LLC to do business in the state you plan to conduct the activity (foreign file). These requirements will greatly increase your cost of doing business.
Another issue with Series LLCs is that since they are so new we lack adequate case law on whether or not the Series LLC will be respected in the state the LLC is filed to do business.
I know this is a long winded response but I hope it helps.
Greg