The answer depends on whether the form "For each ..., A unless B" (as found in Read the Runes) expresses an action that happens simultaneously.
If it is, you discard simultaneously as many cards as cards drawn with Read the Runes, so that Rielle's ability will trigger and refer to that many cards (provided you didn't discard any cards yet this turn).
If it's not, you discard those cards one at a time (or fewer if you choose to sacrifice permanents), so that Rielle's ability will trigger and refer to only one card (provided you didn't discard any cards yet this turn).
10/4/2004 You draw the cards, then sacrifice any permanents you want to sacrifice, then discard the required number of card from your hand. There is no chance to play any cards in between these actions.
Given that this Oracle ruling provides that all sacrifices are performed before all discards, it would appear the discards are simultaneous. If they were sequential discards, each with an optional sacrifice, one would be able to order sacrifices in-between discards.
If I understand correctly, we have no definitive answer yet, right?
No, it's a bit tricky, but we do have a definitive answer. The Gatherer ruling that davidb32 quotes above gives it.
If the sacrifice option wasn't there, it would be clear for the trained eye to see that you discard all cards simultanously, as is the standard when you have only one verb for that action with the phrase "for each": that phrase only serves to make the count of how many cards to discard, it doesn't denote separate actions. What makes this tricky is the sacrifice option, which is governed by this rule:
118.12a. Some spells, activated abilities, and triggered abilities read, "[Do something] unless [a player does something else]." This means the same thing as "[A player may do something else]. If [that player doesn't], [do something]."
This rule breaks the sacrifice and the discard into two separate actions, but it doesn't need to be more than two just because there are multiple objects involved. You sacrifice what you want to sacrifice, then discard for the remainder of the number. The discard is simultaneous, Rielle triggers for that event and you draw as many as you discarded.
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
With the release of Commander Legends, a new rule, which has taken on the existing rule number C.R. 608.2f, was added to the comprehensive rules to cover "actions taken on multiple players and/or objects", especially actions of the form "For each ..., A unless B", such as found in Read the Runes. Since "A unless B" "can't be processed simultaneously, it's instead processed considering each affected player or object individually", as further described in new C.R. 608.2f.
Thus, for example, in the case of Read the Runes, the controller of that spell does the following for each card drawn with Read the Runes (see also C.R. 109.5):
They choose a permanent other than Read the Runes they control.
They then choose whether to sacrifice that permanent, and discards a card if they don't (C.R. 118.12a).
In view of the new C.R. 608.2f, the ruling mentioned in comment 3 is arguably obsolete now, at least in part.
As a result, you discard cards with Read the Runes one at a time (or fewer if you choose to sacrifice permanents), so that Rielle's ability will trigger and refer to only one card (provided you didn't discard any cards yet this turn).
The answer depends on whether the form "For each ..., A unless B" (as found in Read the Runes) expresses an action that happens simultaneously.- If it is, you discard simultaneously as many cards as cards drawn with Read the Runes, so that Rielle's ability will trigger and refer to that many cards (provided you didn't discard any cards yet this turn).
- If it's not, you discard those cards one at a time (or fewer if you choose to sacrifice permanents), so that Rielle's ability will trigger and refer to only one card (provided you didn't discard any cards yet this turn).
See also this thread.EDIT (Nov. 29, 2020): See comment 6.
Given that this Oracle ruling provides that all sacrifices are performed before all discards, it would appear the discards are simultaneous. If they were sequential discards, each with an optional sacrifice, one would be able to order sacrifices in-between discards.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules
If the sacrifice option wasn't there, it would be clear for the trained eye to see that you discard all cards simultanously, as is the standard when you have only one verb for that action with the phrase "for each": that phrase only serves to make the count of how many cards to discard, it doesn't denote separate actions. What makes this tricky is the sacrifice option, which is governed by this rule:
This rule breaks the sacrifice and the discard into two separate actions, but it doesn't need to be more than two just because there are multiple objects involved. You sacrifice what you want to sacrifice, then discard for the remainder of the number. The discard is simultaneous, Rielle triggers for that event and you draw as many as you discarded.
Thus, for example, in the case of Read the Runes, the controller of that spell does the following for each card drawn with Read the Runes (see also C.R. 109.5):
In view of the new C.R. 608.2f, the ruling mentioned in comment 3 is arguably obsolete now, at least in part.
As a result, you discard cards with Read the Runes one at a time (or fewer if you choose to sacrifice permanents), so that Rielle's ability will trigger and refer to only one card (provided you didn't discard any cards yet this turn).