My current King has a personal Stewardship of 66 and a state Stewardship of 95. Note that he's a "Misguided Warrior"... so he gets no Stewardship boost from Education.
How high can it go? If the Devs used a signed single-byte integer for the variable, it will break beyond 127. An unsigned single-byte integer can reach 255 before breaking. Two-byte integers or floating-point numbers are effectively unbreakable without cheats or file editing.
Does anyone know the scope of this particular variable?