Baring exceptional circumstances, everyone needs one executor plus a back-up. Most commonly, this comes down to your spouse plus one other person.
In the event your named executor refuses to act or passes away before you, your back-up would become your executor.
If you haven’t named a backup executor, or if both the original and backup executor have passed away, the law provides for a “chaining” of executors. This means the executor of your executor becomes your executor!