Based on linta's figures
Martials - including monk, rogue, etc 0-50% depending on violence in society. Can't train into any but fighter / barbarian
Wizard: 0.1% of the population (elsewhere) 1% of the population (Cheliax) and possibly as much as 4% depending if including all laundry sorcerers and dropouts
Witches, alchemists, oracles, and similar are rare
Sorcerer: 0.5% (rarer than being a wizard), 3% nobility.
Bard 0.25% (rarer than being a sorcerer)
Rangers 0.25% (same as bards)
Clerics 0.2-1% clerics
Druids: Very low as a % I think
Paladin varies enormously by society but aren't that common and also most members of paladin orders are not mechanically paladins