Based on linta's figures

Martials - including monk, rogue, etc 0-100% 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

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