All the Pope Stephens have two numbers by them because Stephen II from 752 A.D. died four days after taking office, but before he could be officially consecrated.
Because of this, he was left off the official pope charts until 800 years later when church historians decided he belonged there after all.
Unfortunately, this caused confusion because eight other Stephens had held office since his death.
Papal scholars assigned a parenthetical second number to each Pope Stephen after Stephen II. In other words, Pope Stephen V then became Pope Stephen V (VI).
They hoped that this new system would clarify the situation. Obviously, it hasn’t. This is why you have a plan B for everything.