When you advertise goods for sale, make every effort to have enough on hand to supply the demand that it's reasonable to expect. If you don't think you can meet the demand, state in your ad that quantities are limited. You may even want to state the number of units on hand.
State law may require merchants to stock an advertised product in quantities large enough to meet reasonably expected demand, unless the ad states that stock is limited. California, for example, has such a law. In other states, merchants may have to give a rain check if they run out of advertised goods in certain circumstances. Make sure you know what your state requires.