Stacking normally refers to an option you can select for uninsured motorist bodily injury (UM) and/or underinsured motorist bodily injury (UIM) coverages. (See "Uninsured motorist: What you need to know")
Stacking uninsured/underinsured motorist policies is an option that allows you to increase the limits you select for your UM/UIM bodily injury coverage. Limits increase based on the number of cars you are insuring. Keep in mind this increased level of protection typically will raise your insurance premium.
An example of stacking is John has limits of $100,000/$300,000 for his uninsured motorist bodily injury coverage and is insuring two vehicles. If he leaves them "unstacked," or non-stacked as you called it, John's limits would stay at $100,000/$300,000. If John instead chooses to "stack" his UM coverages then his limits would double to $200,000/$600,000.
So by selecting stacking for your uninsured/underinsured motorist bodily injury coverage, you increase your limits for each of these coverages by the number of cars you're insuring. Generally, there is not a limit on the number of vehicles that can be insured and thus stacked this way.
The Property Casualty Insurance Association of America (PCIAA) lists nearly 30 states whose statutes, rules, and/or case law either do not address the issue or specifically allow stacking. However, in many states that allow stacking, auto insurers are permitted to include policy language that prevents policyholders from stacking UM/UIM coverage. So while your state might permit stacking, if your policy explicitly forbids it, you will not be able to stack your benefits.
Since laws in each state vary widely for UM/UIM stacking in accordance with each situation and sometimes depending upon case law plus state insurance laws are continually being changed and update, it is best to check with your state's insurance regulator as well as your insurance agent to find out if you can stack your UM/UIM benefits.
Since "stacking" is the application of more than one policy limit to the same loss or occurrence, if you only have one vehicle on your policy than there wouldn't be coverage to stack. So, if you have a single car policy there is not multiple coverages to stack, and thus stacking would not normally be an option available to you.