A deity's strength is not based on the number of souls, but on influence with the living
In past editions of D&D, a deity's power is based on the number of living worshipers they possess, not the number of souls who have passed to it in the afterlife. While D&D 5th edition has fewer sourcebooks dealing with this lore, it doesn't describe deities in general becoming any more powerful by the acquisition of souls.
Dungeon Master's Guide p.11, "Divine Rank", says that gods have ranks, and that their rank is based on their influence in that world, with gods worshiped on multiple worlds potentially having different ranks in different worlds. This is the only information I can find in 5e-specific sources on what gives deities their rank, and therefore their power level relative to other gods.
Dungeon Master's Guide p.24, "Bringing Back the Dead", notes that the souls of the dead travel to the plane of their deity. Page 63, "Hades", notes that gods and fiends can claim souls, while some are claimed by no deity and loiter on a particular plane.
Mordenkainen's Tome of Foes, p.6, notes that devils recruit mortals into cults, so that they get to claim the soul and transmogrify them into new devils. It's not clear from this book whether deities also do this with souls. Asmodeus (Sword Coast Adventurer's Guide, p.25) does this, but Asmodeus is an archdevil.
Mordenkainen's Tome of Foes, p. 86, notes that Vlaakith absorbs the souls of gith and becomes more powerful. However, Vlaakith is a lich queen, not a deity, and deities in general are not described as sharing this power.
Sword Coast Adventurer's Guide, p. 20, notes that souls may be reborn into the world to finish work, but this is not common. Some who believe in no deity are built into the Wall of the Faithless by the god Kelemvor.
Sword Coast Adventurer's Guide, p.25, describes potential afterlives as "eternally basking in the light of Lathander or endlessly swinging a hammer in the mines of Moradin". This suggests that those good deities do not absorb the souls of the faithful, but keep them around.
D&D 3e's rules were that a deity's strength was explicitly linked to the number of living worshipers. The 3e Forgotten Realms Campaign Setting, page 232, notes:
Because they lose strength if their worship dwindles away and is forgotten, deities task their clerics and others to whom they grant divine spells with spreading their praise and doctrine, recruiting new worshipers, and keeping the faith alive. In exchange for this work and to facilitate it, deities grant divine spells.