Adding uninsured/underinsured motorist insurance to your policy is a smart move. You never know when an accident will happen and everyone wants to have the best coverage possible. In some states, you can stack your coverage in order to have the greatest amount of coverage possible.
You would be able to buy a policy that offers $25,000 coverage per car that you own. So, if you have three cars, then you would have $75,000 of coverage. Of course, the extra coverage comes at a cost. You will see a higher premium each month. Once you balance that cost against how quickly major medical bills can add up to $75,000, you can see that the extra premium is worth it.
Stacking is not legal in all states. In others a claimant has to sue their own insurance company to get the coverage enforced. Talk to your agent about stacking or get a car insurance quote online before you assume that your policy allows for it.