I got this from the
Word of the Titans page in the wiki.
6011549
Higher levels cost more, on a kind of exponential scale that may vary by a large number of factors including the type of unit leveling, the type and number and levels of units croaked, and other activities involving the leveling unit's special abilities that may not even constitute combat. But the main thing is that you should imagine something like a logarithmic curve (not saying it literally is logarithmic), whereby low levels are achievable with a little combat, but levels above 10 or 11 become extremely difficult to obtain, and something like a level 20 unit is completely unheard of. Also notable is that not all of the numbers are available to the units and commanders to calculate, and so the existence of a predicable system of leveling at all is in the realm of the theoretical Mathamancers. Units cannot actually predict when they are going to level (Jillian is being flippant when she tells the Archons, "Cmon, it'll be fun. You'll level." but it would be a reasonable bet they might if they took out a stack of dwagons.)
So there is technically no maximum level for erfworlders, just like there is technically no maximum age for earthworlders (I'm not suggesting that the two have a correlation).
Also, it states
here that Parson is level two. But a lot has happened since that klog was written, so it is possible that Parson has leveled up since then.