Pennant loot mechanism by way of example:
Say you're Briton and you attack a Saracen with
4000 Saracen, 2000 Briton, 1000 Frank pennants. Total pennant loot is about 30% capped by around 520 with adjustments for crown difference and power level difference.
In this case, say 511 pennants are available and you loot all of them. Then you'll gain 511 Saracen Pennants.
Saracen Friend will lose 511 pennants, but not all Saracen- they'll be lost in the following ratio:
Lost Penants * Ratio of pennants of a civilization wrt total pennants.
So Saracen loses
Saracen Pennants = 511 * 4000 / 7000 = 292
Briton Pennants = 511 * 2000 / 7000 = 146
Frank = 511 * 100 / 7000 = 73