Peter believed there were only 8 souls saved at the time of the global flood.
1 Peter 3:20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
Jesus confirmed that Noah and his family were the only survivors as He plainly stated that the flood destroyed them all just as He said the same for those in Sodom.
Luke 17:26 And as it was in the days of Noe, so shall it be also in the days of the Son of man. 27 They did eat, they drank, they married wives, they were given in marriage, until the day that Noah entered into the ark, and the flood came, and destroyed them all.
28 Likewise also as it was in the days of Lot; they did eat, they drank, they bought, they sold, they planted, they builded; 29 But the same day that Lot went out of Sodom it rained fire and brimstone from heaven, and destroyed them all.
So it is not an assumption when it is based in & confirmed in scripture.