How are changelings made?
Its simple.
The changelings take a pony and put it in a cocoon. Then the pony wakes up as a changeling, and the queen will remove its memories.
When Queen Chrysalis, the 152,299,028th queen of the changeling, runs out of guards what will she do?
Get more guards of course, as the most powerful and beloved ponies of the land - the Holders of Harmony. AKA, the Mane 6.