If I remember right, my dad would clean out the extra queen cells and that is where the royal jelly will be located. Royal jelly is fed to the larva to make a regular worker be turn into a queen bee. If your hive has extra queens that hatch, then your hive will split and swarm usually taking most of the honey with them to find a new home, so its important to clean out these extra queen cells to prevent this from happening. I imagine if you wanted to produce more royal jelly you could add some of your existing royal jelly to new brood cells and the worker bees will continue to add royal jelly to these cells. Before the larva mature you can extract the royal jelly from them.