There's no explicit guidance
The Casting Spells section (Mini Six Cinematic Roleplaying System p10) says:
All spells have a target number (TN) listed in their description. Sorcerers roll Magic to determine success. Only one spell may be cast per turn. Success on the roll mean[s] the spell is cast, see each spell description for the exact effect achieved. On a failed roll, the Sorcerer suffers a -1D penalty to their Magic skill. Further failures increase the penalty. One hour of rest with no interruption removes all penalties.
The slumber spell (13), along with the illusions spell (12) are the only spells listed in the book with multiple targets where the Resist Roll modifies the Target Number.
Neither spell remarks at all about the potential for multiple failures costing more dice. The Casting Spells rules section states that only one spell may be cast per turn. That coupled with the description of the illusions spell stating:
GMs may wish to roll for the highest Resist first and if they fail consider all to have failed.
suggests that the intent of the rules is that one spell resisted is one failure. The line stating "Further failures increase the penalty", seems to be talking about casting again, but it's not definite.
This is neither explicit nor strongly implied in the rules, so there must be a decision by the GM.
I'd rule that for slumber the sorcerer rolls once, and each target rolls once. Then if the sorcerer fails to affect any targets at all, then the spell failed, and he's penalized by one die until he can rest for an hour (the stated recovery mechanic. But if he succeeded versus one of the targets, then the spell was a success.
Alternately, the GM could have the magic Die loss happen if any number of targets resist the spell, whether or not any other targets were affected, as that could be seen as a failure as well.
Ruling the other way, that each resistance is an individual failure, which is also a reasonable reading of what's written ("Further failures increase the penalty"), makes each of those two spells quite risky to use; penalizing the sorcerer for targeting more creatures. That said, he can choose to only target one creature, and only risk one Magic Die.