One debate is how often a cleric can turn undead. The DMG states that the cleric’s turn attempts are limited (p. 65, “As stated on the CLERICS AFFECTING UNDEAD TABLE, this function may be attempted only once by each cleric”). Clearly “may be attempted only once” doesn’t mean, “ever, in the lifetime of the cleric”, so the text must be examined for the limiter; it’s not purely “once”, nor “always”.
One might contend that turning is limited to once per encounter, requiring defining “what is an encounter”. Defining by encounter requires house ruling the exceptional cases. What if an encounter last longer than the turning effect of 3-12 rounds? What happens in the middle of the fight if more undead arrive (one encounter becoming two or more concurrent encounters?). What if the characters break away from the encounter (closing the door for example where skeletons won’t follow them), only to wait a few rounds, and reopen the door for a new encounter? While turning is discussed in the context of an encounter, nowhere does the text define the number of times a cleric can turn in the context of an encounter.
Once per encounter significantly nerfs the cleric’s turn ability against large groups of same-type undead (where large is groups greater than 12). Once per encounter doesn’t explain from a game perspective why a cleric can only keep turning the 100 skeletons only if there’s a zombie present (“Companion, raise a zombie so I can keep turning those skeletons we know are coming”). What well-intentioned necromancer would mix undead types?
I contend that the limit is once per individual undead, more specifically limited by failure to turn individual undead. If the cleric fails to turn undead, they can’t try again against those undead, but they can continue turning any remaining undead as long as they don’t fail. That’s supported by the text: “Any failure to turn undead disallows a further attempt by the same cleric” — failures limit attempts, successes do not.
That sentence is followed by, “Turning can occur at the same time as missile discharge, magical device attacks, and/or spell casting” and “It also is subject to initiative determination”, neither of which is relevant or specific to multiple forms (nor in need of clarification); instead further clarifications of the overall subject of turning undead. Looking at the entirety of the text: Turning undead “may be attempted only once by each cleric … Any failure to turn undead disallows a further attempt”.
The actual limiter of failure is consistent with other aspects of turn undead. When counter-affecting undead the cleric can keep counter-affecting undead “indefinitely until one or the other cleric fails and is no longer eligible to affect undead.” Clerics can re-turn undead that have returned after 3-12 rounds (“The turned undead will be able to come back again, but they are subject to further turning by the cleric.”) — those they have not failed against. Evil clerics can gain control of undead by not failing, the actual duration being defined by the level of their success. In the case of a “D”, they can repeatedly renew every 6 days, as long as they don’t fail. The limits for all of those are failure.
Success is defined as any case the cleric doesn’t explicitly fail. If a cleric attempts to turn a group of undead –single or multiple types– and fails, the cleric has failed against all of them (if he’d succeeded at all, he’d be determining which undead in the crowd were turned). “Failure to score the number shown, or greater, means the turning was unsuccessful.” If the cleric tries against a group, and succeeds, that’s not a failure even if some of the undead were not turned on a d12. The cleric hasn’t failed against the remaining undead yet (“Any failure to turn undead disallows a further attempt”), and could try again.
That then is consistent with the discussion of multiple types. Multiple types of undead create two cases that could cause exceptions to the base rules:
1) “what happens if the roll succeeds against one type, but fails against the other(s)?” If the cleric suceeds against any type, the net result is not a failure, and thus the cleric can try again, as long as he keeps turning something with each attempt, as he’s continued to not fail.
2) “what happens if the roll succeeds against all the multiple types”, which is clarified as they are turned “from the lowest HD first”.
There is the option presented to the DM of defining failure to instead be failure to turn all of them (must roll enough to turn the vampire to turn the ghasts and ghouls), making packs of undead more lethal, further supporting that the limit is failure.
Clerics can fail to turn undead once; until then they can try again. In the case of the exceptional case of multiple types, failure is clarified as failing to turn all types, not failure to turn some types.
One could choose to house rule “once” to additionally mean “fail once / encounter” or “fail once / day|week” or “once / level.” But it’s still … once.