Fifteen, as in Genesis 7:20, signifies to be very few so as to be almost nothing.

(References: Arcana Coelestia 792)

792. Verses 19-20 And the waters grew stronger and stronger over the earth, and all the high mountains beneath the whole sky were covered. For fifteen cubits upwards the waters prevailed and covered the mountains.

'The waters grew stronger and stronger over the earth' means that persuasions of falsity increased in this way. 'And all the high mountains beneath the whole sky were covered' means that all goods stemming from charity were done away with. 'From fifteen cubits upwards the waters prevailed and covered the mountains' means that no trace of charity remained, 'fifteen' meaning things so few as to be scarcely any at all.