while browsing through the ElementalDefs,xml a while a go i found those two lines
<UnitLevelUpPerkOfferCount>5</UnitLevelUpPerkOfferCount>
<CityLevelUpPerkOfferCount>3</CityLevelUpPerkOfferCount>
they seem to be what you want, but i haven't played around with them yet. Consider this a starting point 
edit: you are too quick for me parrottmath 
