"Seal Fragments" 

Maybe, other milder gates only had five or ten otherworldly people's souls which could last for hundreds of years or thousands of years if the energy in the soul was used sparingly. 

The Godfather said that his soul had been trapped for hundreds of years, yet because his soul was relatively strong, up to now,  it still had a few breaths left. 

However, weaker souls that were used to seal these highly dangerous gates must have been used up, and one wonders how the world Union replenished the lost souls. 

There was an even more bizarre question here. 

Just how many otherworldly people's souls were summoned to this world each year? 

The number must be at least ten or more. 

Just say that all abyss gates in this world needed thousands of souls once every hundred years on average, which means, the world union indeed needed around ten souls per year.