I've never seen or been told of a maximum amount that can be removed from a rocker arm face, Kansas.
Assuming that you're using a valve grinding machine with the appropriate attachment for resurfacing the rocker arm face of shaft mounted rocker arms, the attachment will cause the rocker arm face to be guided during the re-surfacing process so that the face of the rocker arm is appropriately radiused for actuating the movement of the valve stem smoothly..
Some y-block Ford rocker arms that I've re-surfaced had significant valve stem indentations where the valve stem had moved against the rocker arm face. They cleaned up with the re-surfacing process and the rocker arm adjuster seemed to compensate adequately for the lost material. Adjusting the rocker arms was MUCH easier after the re-surfacing process AND my feeler gauge no longer got beat-up.
I see re-surfacing the rocker arm face as being akin to grinding an engine valve face at a 45 degree angle. For every .002 removed from the 45 degree angle valve face you reduce the valve stem length by .001 to reduce its length and compensate for the valve protruding farther through the valve guide (i.e., closer to the rocker arm face). In the case of the rocker arm, the rocker arm adjuster is the compensator for the material you removed that moved the rocker arm face away from the valve stem contact end. -In essence, turning the rocker arm adjuster 'in' serves to lengthen the pushrod.
In high performance or high operating rpm applications, some of this likely gets looked at more carefully in that valve train geometry may become an issue as rocker arms are re-surfaced multiple times.
Hope this helps.
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York