That's not mathematics, it's vibes.
The players signed under him (with x next to their name if they're still in the club roster)
Ederson (x)
Dias (x)
Gundogan (x)
Walker (x)
Silva (x)
Stones (x)
Laporte (x)
Mahrez (x)
Rodri (x)
Grealish (x)
Akanji (x)
Haaland (x)
Phillips (x)
Ake (x)
Mendy
Sane
Zinchenko
Gabriel Jesus
Joao Cancelo
Ferran Torres
Zach Steffen
Nolito
Angeliño
Bravo
Danilo
14 out of 25 signings are still at the club. The ones who aren't: Zinchenko, Gabriel Jesus, Sane, Torres all chose to leave. Cancelo had disciplinary issues of some kind. Steffen is just a backup GK (and is still technically at the club). Mendy is still at the club but I'm not gonna count him. Him, Danilo, Bravo, Angeliño, were all moved on.
Some of the departed players had long stints, Sane was there 3 years (really 4 but the whole injury thing is misleading), Cancelo 3.5, Jesus 6, Zinchenko 5.
As for departing players, many left because they were getting older or were past it (Demichelis, Nasi, Mangala, Touré, Silva, Aguero, Kompany, Otamendi, Delph). Some were simply not good enough. Sterling left for money/gametime.